modelling hydrological responses to land use and climate change: a southern african perspective

BioOne sees sustainable scholarly publishing as an inherently collaborative enterprise connecting authors, nonprofit publishers, academic institutions,research libraries, and research funders in the common goal of maximizing access to critical research.

Modelling Hydrological Responses to Land Use and Climate Change: ASouthern African PerspectiveAuthor(s): Roland E. SchulzeSource: AMBIO: A Journal of the Human Environment, 29(1):12-22. 2000.Published By: Royal Swedish Academy of SciencesDOI: http://dx.doi.org/10.1579/0044-7447-29.1.12URL: http://www.bioone.org/doi/full/10.1579/0044-7447-29.1.12

BioOne (www.bioone.org) is a nonprofit, online aggregation of core research in the biological, ecological,and environmental sciences. BioOne provides a sustainable online platform for over 170 journals and bookspublished by nonprofit societies, associations, museums, institutions, and presses.

Your use of this PDF, the BioOne Web site, and all posted and associated content indicates your acceptance ofBioOne’s Terms of Use, available at www.bioone.org/page/terms_of_use.

Usage of BioOne content is strictly limited to personal, educational, and non-commercial use. Commercialinquiries or rights and permissions requests should be directed to the individual publisher as copyright holder.

http://dx.doi.org/10.1579/0044-7447-29.1.12

http://www.bioone.org/doi/full/10.1579/0044-7447-29.1.12

http://www.bioone.org

http://www.bioone.org/page/terms_of_use

12 © Royal Swedish Academy of Sciences 2000 Ambio Vol. 29 No. 1, Feb. 2000http://www.ambio.kva.se

INTRODUCTIONLand use change and climate change form a complex and inter-active system by linking a human action, viz. the land use change,to environmental reactions which, in turn, impact again on hu-man responses. Further complicating this system is the fact thatthese linkages occur at different spatial and temporal scales (1).A major environmental reaction to land use change occurs in hy-drological responses such as changes in runoff components, ero-sion or groundwater recharge rates, with these responses beingfurther complicated when accompanied by any changes in cli-mate, be they short or long term.

This paper addresses issues outlined above by reviewing re-sults from field observations and hydrological simulation mod-elling to a range of scenarios of land use and climate changesin a southern African context. Significant in the context of south-ern Africa and most of the rest of the continent is that,hydrologically, problems are perceived and experienced at alocal rather than at a national to global spatial scale, and at anintra- to inter-seasonal rather than on a decadal time scale. Fur-thermore, the emphases in the International Geosphere-BiosphereProgramme’s (IGBP) core initiatives on modelling, hydrologyand land use change, namely the GAIM, BAHC, GCTE andLUCC programmes, often seem far removed from the harsh re-alities of the lives of many Africans whose day-to-day encoun-ter with climate and climate change often focuses on having tofind enough water for the week and store it, as well as produc-ing enough food to last through the next dry season.

The core of the paper consists of nine issues, or hypotheses,which are illustrated with examples from southern Africa. Theseissues are:Issue 1. Southern Africa’s hydrological regime is already so

highly variable in space and time, that climate changetrends may be difficult to detect.

Issue 2. Fluctuations in the hydrological regime are amplifiedand exacerbated by fluctuations in climate.

Issue 3. Hydrological responses are highly sensitive to, and de-pendent upon, land use and its change.

Issue 4. Abrupt land use changes at local scale may behydrologically far more significant than gradual landcover changes at regional to global scale.

Issue 5. Changes in land use frequently exacerbate already vari-able flow regimes.

Issue 6. The detail of spatial information may be vital in as-sessing hydrological responses of critical land uses.

Issue 7. Between one region and the next, major componentsof the hydrological system often respond very differ-ently when subjected to climate change.

Issue 8. Hydrological concerns in developing countries are cur-rently focused more on inter-seasonal scales than ondecadal scales of climate change.

Issue 9. In order to be proactive in regard to long-term climatechange, there is a need to identify hydrologically sen-sitive areas.

A short section on terminology used and techniques applied pref-aces the discussion of the nine issues.

TERMINOLOGY AND TECHNIQUESHydrology may be defined as the interdisciplinary geosciencewhich deals with the processes governing the replenishment anddepletion of terrestrial water resources (2). It revolves aroundunderstanding and describing quantitatively the various physi-cal, chemical and biological components and processes whichinteract and operate at a wide range of scales in time and spacein an already complex land phase of the hydrological cycle, ren-dered even more complex by conscious and unconscious humanalterations to the hydrological system, for example, by the con-struction of dams or changes in land use.

Land cover (Fig. 1) refers to the biophysical state of part ofthe earth’s surface and immediate subsurface in terms of broadcategories such as cropland, forest, grassland, settlements, rec-

Modelling Hydrological Responses to Land Useand Climate Change: A Southern AfricanPerspective

Nine hydrological issues relating to land use and climatechange are identified from a southern Africa perspective,each illustrated by an example based on field observationsor simulation modelling. The nine issues are that (i) south-ern Africa’s hydrological regime is already so variable thatclimate change will be difficult to detect; (ii) fluctuations inthe hydrological regime are amplified by fluctuations inclimate; (iii) hydrological responses are highly sensitive toland use changes; (iv) local scale abrupt land use changesmay be hydrologically more significant than regional scalegradual changes; (v) land use change frequently exacer-bates already variable flow regimes; (vi) detailed spatialinformation is vital in assessing impacts of critical landuses; (vii) major components of the hydrological systemrespond very differently to climate change; (viii) in devel-oping countries inter-seasonal climate change may bemore important than that at decadal time scale; and (ix)there is need to identify the hydrologically sensitive areasof a region.

Roland E. Schulze

Figure 1.The relationshipbetween land coverand land use.

Waterbodies

13Ambio Vol. 29 No. 1, Feb. 2000 © Royal Swedish Academy of Sciences 2000http://www.ambio.kva.se

reation, water bodies or mining (1). These broad landcover cat-egories can be changed by natural factors such as long-term cli-mate changes or climatic persistence—for example, consecutiveyears of drought— or naturally occurring episodic events suchas fire or flooding. Overwhelmingly, however, land cover hasbeen changed by human actions through land cover conversionand modification, primarily for purposes of agricultural produc-tion and settlement (1). These conversions and modifications ofland cover introduce the concept of land use as distinct from landcover (Fig. 1). Land use refers to the manner in which biophysi-cal attributes of the land are manipulated, managed and ex-ploited. Land use thus refers, inter alia, to the utilization, hu-man inputs and management levels, driven by changing produc-

in the context of hydrological responses in southern Africa.Hydrological impacts of land use and climate change are of-

ten evaluated by means of simulation models. In broad concept,a hydrological model is a quantitative expression of observation,analysis and prediction of the interactions of the various hydro-logical processes which vary in time and over space, i.e. rain-fall, infiltration evaporation or streamflow. Because the naturaland human hydrological systems e.g. modified by land-usechange, are complex ones, the models are simplifications of thebehavior of hydrological responses (4, 5).

In most modelling examples illustrating this paper the ACRUagrohydrological simulation model has been applied. ACRU isa deterministic, physical-conceptual (i.e. process based, relating

Figure 3. An example of averification study with the ACRUmodel taken from the Lions riverin the Mgeni catchment (6).

tion and consumption dynam-ics, and is subject also to so-cial, political and economicfactors. Manifestations of landuse and its management mayhave significant hydrologicalresponse impacts by either en-hancing or retarding infiltra-tion, thereby reducing or en-couraging stormflow genera-tion and its resultant changes insediment and/or nutrient pro-duction into watercourses.

In regard to climate change,two forms may be distin-guished (3). The first is climatechange which encompasses allforms of climate inconsistency,where deviations from long-term statistics take place overan area in the knowledge thatover time these inconsistenciesare reversible and non-perma-nent; eg. the El Niño phenom-enon. Climate change, thus de-fined, is an entirely naturalphenomenon which has takenplace many times in history.The second form of climatechange encompasses thosechanges in, say, temperatureand precipitation which are ir-reversible, with new and per-manent trends in climate statis-tics, i.e. with signals of a dis-tinct sign being superimposedon natural variability; e.g.change resulting from the en-hanced greenhouse effectcaused largely by human ac-tions. This paper will addressboth forms of climate change

Figure 2. The ACRU modellingsystem: Structure (4).

PRECIPITATION(RAINFALL; IRRIGATION)

CANOPYINTERCEPTION)

SURFACE LAYER

SATURATION . . . . . . . . . . .DRAINED UPPER LIMIT . . .STRESS THRESHOLD . . . .WILTING POINT . . . . . . . . .

SUBSEQUENTSOILHORIZONS

SPECIFIC YIELDHYDRAULIC CONDUCTIMITYHYDRAULIC GRADIENT

GROUNDWATER STORE

RUNOFF

ST

OR

MF

LOW

ST

OR

E

INTERMEDIATE STORE

CAPILLARY FRINGE

BASEFLOW

QUICKFLOW

TO

TA

L E

VA

PO

RA

TIO

N(A

CT

UA

L E

VA

PO

RA

TIO

N)


cause and effect), and integrated multipurpose modelling sys-tem (4), revolving around a daily time step multilayer soil wa-ter budget (Fig. 2). Individual internal state variables (e.g. soilmoisture) as well as end-product output from the model (e.g.streamflow or sediment yield) have been widely verified in ex-perimental field and catchment conditions under different hydro-logical and land use regimes in Africa, Europe and the Ameri-cas (4). One example of a verification study on an operationalcatchment with mixed land uses is illustrated in Figure 3, inwhich flows from the 362 km2 Lions river in KwaZulu-Natalprovince of South Africa are simulated with ACRU. Not only isa good visual fit of the time trend evident, but accumulated flowsand monthly flows in the wettest year in 10 (90th percentile ofexceedance) and the driest year in 10 (10th percentile) are alsomimicked well and the indices of model performance show agood fit for a range of critical statistics (6).

In the ACRU model total evaporation is partitioned into soil

Issue 1. Southern Africa’s hydrological regime is already sohighly variable in space and time, that climate change trendsmay be difficult to detect.Figure 4 (top) shows the distribution of simulated median an-nual runoff (that is, stormflow plus baseflow) over southern Af-rica. For this simulation the region was delineated into the 1946relatively homogeneous Quaternary Catchments as identified bythe South African Department of Water Affairs and Forestry. Toeach Quaternary Catchment a representative rainfall station with45 years of concurrent and checked daily rainfall data (1950 -1994) was assigned, as was information on monthly tempera-ture and reference potential evaporation, hydrological soil prop-erties and relevant vegetation characteristics. For this simulationa land use of grassland, taken to be under good grazing man-agement, was assumed. All spatial attributes were geocoded ontoa geographic information system (GIS) to which the model andclimatic databases were linked, with the Quaternary Catchments

water evaporation and transpiration,thus rendering it sensitive to tempera-ture change as well as accommodatinga transpiration suppression functionassociated with increases in CO2.In runoff generating routines, accountis taken of land use/tillage inducedchanges in initial infiltration and soilwater redistributions as well as of rain-fall characteristics, while the stormflowand baseflow components of runoff aremodelled separately and explicitly.Detailed descriptions of processes andoptions have been given elsewhere (4).

ISSUES RELATED TO LANDUSE AND CLIMATE CHANGEDRIVEN RESPONSES INHYDROLOGYNine issues, or hypotheses, are pre-sented below to illustrate some of thecomplexities of the interactions of hy-drology, land use and climate change,as experienced from a southern Afri-can perspective. Case studies, basedboth on field observations as well asfrom modelled results are taken froma range of actual catchments as well asfrom southern Africa viewed as a sin-gle region. Where the term southernAfrica is used in context of this paperit implies the contiguous area compris-ing of South Africa plus Lesotho andSwaziland.

Figure 4. Distributionof median annualsimulated runoff (top)and the coefficient ofvariation (%) of annualrunoff (bottom) insouthern Africa (7).


considered to be responding as individual hydrological entities.Spatially the median annual runoff (MAR) is highly variable,

ranging from < 5 mm equivalent runoff to over 250 mm. Whilea general decrease in MAR is evident inland of the southern andeastern coastal zones the distribution is, furthermore, spatiallycomplex with often catchments of high and low MAR in veryclose proximity, usually as a result of major rainfall changes as-sociated with physiographic discontinuities.

This high risk hydrological environment of southern Africais highlighted even more by examination of Figure 4 (bottom),which illustrates the inter-annual coefficient of variation (CV,%) of runoff (7). Half of southern Africa has a CV of annualflows in excess of 100% and even the wetter eastern regions stillhave CVs around 60%. Within a year the month-by-month CVsof runoff are even higher. Such features of spatial and temporalhydrological responses under present conditions render regionalwater resources development very difficult and expensive.

ern African dams characteristically operate at only a fraction oftheir full supply capacity much of the time, any prognostic aidin dam-operating decisions, such as El Niño forecasts, could po-tentially save the subcontinent vast sums of money. Figure 5 il-lustrates the amplification of runoff response relative to rainfallresponse. In this study, daily observed rainfalls and associatedsimulated runoff responses were isolated for the 1982/83 hydro-logical year—1 October to 30 September—which represented avery strong El Niño, from each of the 1946 Quaternary Catch-ments for which daily hydrological simulations from 1950–1994were being made. The rainfall as well as runoff for 1982/83 werethen expressed as ratios of their respective long-term (45 year)median annual values for each catchment.

Perusal of Figure 5 (top) shows that over much of the sum-mer rainfall regions of the eastern 2/3 of southern Africa the ElNiño year’s rainfall was 50–80% of the long-term median, how-ever with sizeable areas receiving within the range of expected

Figure 5. Ratios of observedrainfall (top) and simulatedrunoff (bottom) for the 1982/83 El Niño season inrelation to 45 year medianannual values in southernAfrica (5).

The uncertain changes in rainfallamounts, seasonality and intensitypatterns as well as rainday per-sistences induced by any climatechange are likely to result in anymeaningful trends in runoff beingvery difficult to actually detect bywater resources planners in southernAfrica because the high inter- and in-tra-seasonal “noise” could easilymask any “signal” in runoff changeresponses.

Issue 2. Fluctuations in the Hydro-logical Regime are Amplified and Ex-acerbated by Fluctuations in Climate.At the timescale of reversible climatechange, southern Africa’s strong sea-sonal climatic rhythms have beenshown to be severely subjected to ElNiño-related influences. In southernAfrica an El Niño generally impliesa regional drought. Repercussions oflow rainfalls during El Niño eventsare usually amplified in the water sec-tor and major water resources opera-tors, electricity utilities who operatewater-cooled power stations or irriga-tion boards are frequently called uponto make far-reaching and costly deci-sions as to when, for example, to in-troduce water restrictions for human,industrial or agricultural purposes orwhen to switch over to interbasin wa-ter transfers. Since many major south-


rainfalls (80–120%), while a small fraction received only 20-50%of the expected precipitation. The corresponding simulated run-off responses (Figure 5, bottom) display much more complexpatterns both spatially and in the range of ratios. Much of theregion only yielded 20–50% of the long-term median runoff,with considerable areas generating < 20% of expected runoffs—showing clearly the intensifying effect of the hydrological cy-cle on rainfall perturbations. Also very evident is the patchinessof runoff responses, illustrating the high dependence of runoffon local antecedent wet or dry catchment conditions, rather thanonly on total amounts of rainfall.

This example has shown clearly how fluctuations in rainfallhave been amplified and exacerbated in the hydrological re-sponse.

Issue 3. Hydrological responses are highly sensitive to, anddependent upon, and use and its change.In many catchments the natural vegetation has been highly modi-fied by a mixture of intensive crop cultivation, urbanization orovergrazing. The 4079 km2 Mgeni catchment in KwaZulu-Na-

and intensification. In a second assessment, the percent-age changes in median annual streamflows resulting from themodifications and conversions of pristine land covers weremapped. Figure 8 (top) illustrates how MAR reductions of upto 61% can occur, mainly in areas of intensive sugarcane andexotic forest plantations, while gains in MAR of up to 103%were simulated in areas which were urbanized or had densepopulations where overstocking and associated land degradationwere prevalent.

Changed hydrological responses to land use are not confinedto runoff changes, however, and in a third assessment Figure 8(bottom) illustrates the spatial patterns of the biological statusof the receiving streams of the Mgeni system in a map of simu-lated mean annual concentrations of the pathogen Escherichiacoli (E. coli). In developing and verifying the model to simu-late the fate of E. coli in a catchment, two land use variableswere identified as major driving forces, namely, livestock den-sity and the number of humans living in close proximity (< 250m) to streams and under conditions of poor sanitation (6). Themap shows simulated E. coli concentrations to range from un-

Figure 6. Distributionof pristine (top) andpresent (bottom) landcovers in the Mgenicatchment (6).

tal province of South Africa isone such highly modified catch-ment in which natural, or pris-tine, vegetation represented bysix of Acocks’ so-called VeldTypes (8) has been replaced by21 classes making up presentland uses and covers (Fig. 6, topvs bottom). After extensive veri-fication studies of the ACRUmodel in the Mgeni catchmenton seven catchments with arange of combinations of landuses, soils and climatic condi-tions (6), the influence of landuses on hydrological responseswas assessed in three analyses.

In a first assessment, the run-off coefficient, i.e. the mean an-nual runoff (MAR) expressed asa percentage of the mean annualprecipitation (MAP), was plot-ted against MAP for 137 sub-catchments delimited within theMgeni catchment. In Figure 7the plot for pristine land coversshows an expected high correla-tion between the runoff indexand MAP, while the runoff co-efficient for present land coversdisplays no association what-soever with MAP, illustratingclearly the sensitivity of run-off to change in the originalland cover due to modification


der 250 (lowest, 30) to over10 000 (highest, 18 200) countsper 100 ml, with areas of high-est concentrations associatedwith informal settlements, cat-tle feedlots and areas of highgeneral stocking rates.

Issue 4. Abrupt land usechanges at local scale may behydrologically far moresignificant than gradual landcover changes at regional toglobal scale.Abrupt changes to the physicallandscape can include devastat-ing episodic events such as afire ravaging an area. In an Af-rican context where both inten-tional burning and wildfires area common feature, reported ef-fects on hydrological behaviorcan be nothing short of dra-matic (9), with often serious en-vironmental and economic con-sequences downstream. In onesuch example the first rainyseason’s post-fire effects ofthe 25 August 1989 wildfire inresearch catchment V1H020(area: 1.32 km2) are assessed.This catchment was afforestedpartially to Eucalyptus fasti-gata, and is nested 800 mdownstream of grassed researchcatchment V1H028 (area: 0.42km2) which did not burn andthus acted as a control catch-ment. These two catchmentsare part of the Ntabamhlopehydrological research stationin KwaZulu-Natal, located at29°50' S, 29°50' E and with aMAP of 980 mm.

While the high intensitywildfire only destroyed theafforested lower 26.5% of thetwo nested catchments, the 10months immediately follow-ing the fire saw stormflows atV1H020 increasing by 92%from the statistically expectedvalues and peak discharges by1100%, while times to peakwere reduced by 53% (9). Fig-

Figure 7. Associations between the runoff coefficient and MAP (mm) for 137 subcatchments of the Mgeniriver system for (a) pristine and (b) present land uses.

Figure 8. Percentagechanges in MAR as aresult of conversion frompristine land cover topresent land use (top)and mean annualconcentrations of E. colifrom present land use(bottom) in the Mgenicatchment (6).

(a) PRISTINE LAND COVER (B) PRESENT LAND USE


ure 9 gives examples of pre-fire and post-firehydrograph responses for the partially burned andthe control catchments (9).

It is events such as the one described above, withtheir immediately experienced hydrological reper-cussions, that are frequently the focus of local con-cern, within and downstream of a catchment, ratherthan larger scale gradual land cover changes overyears or decades with imperceptible hydrologicalchange from one year to the next.

Issue 5. Changes in land use frequently exacerbatealready variable flow regimes.Streamflow is made up of stormflow, which is gen-erated at and/or near the soil surface of a catchmentfrom a specific rainfall event and baseflow, whichconsists of water from previous rainfall eventswhich has percolated through the various soil hori-zons into the groundwater zone and then contrib-utes as a delayed flow to the streams within a catch-ment.

An intensification of land use from, say, an an-nual natural grass cover to (say) evergreen exoticforest or sugarcane plantations implies higher rain-

Figure 9. Examples of pre- and post-wildfire hydrograph responses at theNtabamhlope hydrological research station on unburned and burned portions ofthe instrumented catchments (9).

Table 1. Coefficients of variations of 50 years’ simulated stormflows and baseflows for three land use scenarios inthe Nadi catchment, KwaZulu-Natal (10).

(a) Stormflows

Land use Coefficient of Variation (%)

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Ann

Grassland, good condition 140 165 107 145 374 290 501 282 392 186 151 124 126Grassland, degraded 108 131 88 131 291 290 449 253 343 139 114 99 89E. grandis plantation 173 194 118 181 394 275 509 275 404 223 187 148 159

(b) Baseflow

Land use Coefficient of Variation (%)

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Ann

Grassland, good condition 215 175 177 171 170 169 162 157 162 267 258 213 145Grassland, degraded 240 195 188 189 189 189 182 179 181 301 293 247 165E. grandis plantation 250 183 180 179 179 170 170 160 185 326 244 246 159

fall interception rates from both the canopy and litter/mulch lay-ers, also enhanced infiltration rates as a consequence of tillage,higher transpiration rates as a result of increased biomass andaerodynamically rougher canopies (including year round tran-spiration if the plants to not senesce in the dry season—and soilwater extraction from deeper soil layers as a result of deeper root-ing systems. Consequently, near-surface stormflow generationmay be reduced, as well as less water percolating into thegroundwater zone to feed the baseflow store. Vegetation degra-dation, on the other hand, exposes more soil directly to rainfall,infiltrability may be reduced by crusting of the exposed soil, andtrampling by livestock may further reduce initial abstractionsbefore stormflow commences. This may lead to an increased“flashiness” of the catchment with resultant enhanced peak dis-charges and sediment yield, while the baseflow store is replen-ished to a lesser extent than with good vegetation cover.

The example selected to illustrate that land use change fre-quently exacerbates flow regimes is taken from the 74 km2 Nadicatchment in KwaZulu-Natal (MAP 552 mm), in which subtropi-cal grassland originally in good hydrological condition has, onthe one hand, been degraded by severe overgrazing (10), whileon the other hand parts of the catchment are currently being con-verted to Eucalyptus grandis, a fast growing evergreen exotic

tree species grown for commercial purposes. By altering modelinput in regard to differences in intra-seasonal leaf area index,canopy interception and root distributions as well as infiltrationand stormflow generating variables associated with different landuses, the respective stormflows and baseflows were simulatedfor grassland in good condition, in degraded condition and foran E. grandis plantation. While differences in median annualrunoffs for the three land-use scenarios were as expected, withMAR of grassland in good condition being 61.1 mm, that of de-graded grassland considerably higher at 89.6 mm and of the plan-tation lower at 44.7 mm, it is the increases in the variability offlows—for the different sets of reasons described above—thatare illustrated clearly in Table 1 for stormflows in the case ofplantations, and for both stormflows and baseflows in the caseof degraded grassland and plantations.

Issue 6. The detail of spatial information may be vital inassessing hydrological responses of critical land uses.The significance of land use change on hydrological responseshas already been illustrated clearly in Figures 7 and 8. Some landuses are, however, more critical than others in their impacts ondownstream users, be it because a greater overall reduction instreamflows takes place or because certain seasons’ critical flows


may be impacted more severely by such land uses. When con-flicts between up- and downstream water use sectors arise in acatchment, and simulation modelling is used as a tool to resolvethose conflicts and to optimize equitable allocations of water tocompeting sectors, then the areal extents of the critical land uses,and other associated hydrological attributes of those land uses,have to be assessed at appropriate levels of detail.

In the sub-humid areas of South Africa two such competingwater use sectors are irrigation and commercial afforestation. Ir-rigation abstracts large volumes of water either directly from astream or from an impoundment fed by a stream, thereby reduc-ing the availability of water to downstream users. This reduc-tion is likely to be more critical in low flow months. Afforesta-tion, thereagainst, reduces amounts of rainfall reaching the soilsurface by high canopy/litter interception losses, reducesstormflows by a combination of enhanced infiltration as well astranspiration losses and reduces baseflow production by soil-water extraction from a relatively deep rooting system. As in thecase of irrigation, afforestation to fast-growing evergreen exotic

Figure 10. Identification ofafforested areas in the Bivanesubcatchment at differentlevels of detail (11).

mearnsii) have different growth rates and soil-water extractionpatterns (4), but Figure 10 (middle) shows significant areas ofafforestation which were identified neither by Landsat TM northe USGS methods. In South Africa, Landsat TM imagery is fre-quently used by government and forestry industry as the meansof identifying commercial afforestation. In this case study, theaerial photographs identified 44.8% more afforestation than was“officially” there (cf. Fig. 10 middle vs 10 bottom), with sig-nificant potential influences on local and downstream water re-sources.

In the same catchment, the study of dams revealed thatLandsat TM vs aerial photograph dam surface areas were 39 havs 209 ha, the number of dams was 10 vs 59 (with the govern-ment database only containing 5 dams) while the estimated fullsupply capacities of the dams were 1.43 x 106 m3 from LandsatTM vs 4.18 x 106 m3 from aerial photographs, i.e. a differenceof nearly 3 times (11).

These discrepancies illustrate clearly that the detail of land useinformation is vital in assessing hydrological responses, certainly

species generally impacts low flows relativelymore than high flows.

In the 1261 km2 Bivane catchment inKwaZulu-Natal in South Africa (latitude 27°40'S,longitude 30°45'E) conflict between downstreamirrigation water users and further potential reduc-tions of upstream streamflows by proposed ad-ditional afforestation has resulted in a govern-ment moratorium being placed on any new af-forestation until “afforestation vs irrigation” is-sues have been resolved—by appropriate hydro-logical simulation modelling. This has necessi-tated, inter alia, a detailed inventory of presentday afforestation and of stored water in damswithin the Bivane catchment (11).

Three approaches to assessing the areal extentof present afforestation and characteristics ofdams were examined (Fig. 10). At the coarsestlevel, forests could be identified from the 1 kmx 1 km land use grid supplied by the UnitedStates Geological Survey (Pers comm.). Figure10 (below) shows this resolution to be clearly in-adequate for, while it identifies major areas of af-forestation as “evergreen”, it could not pick upany of the many small dams which exist in thatcatchment.

Even larger discrepancies between respectivepercentages of critical land covers became evi-dent when results from 1:30 000 aerial photo-graphs, supplemented by groundtruthing in thefield, were compared with those from satelliteimagery using Landsat TM. Not only could thephotographs distinguish between the major gen-era grown by commercial foresters, which is im-portant because the genera and main species (Eu-calyptus grandis, Pinus patula and Acacia

Bivane Catchment: Land Cover from USGS-AVRR 1 km 2 Grid(from USGS, using AVRR, April 1992–March 1993


at local scale, and certainly where critical land uses with poten-tially major hydrological repercussions are under scrutiny.

Issue 7. Between one region and the next, major components ofthe hydrological system often respond very differently whensubjected to climate change.A perception exists that under conditions of an enhanced green-house gas atmosphere, shifts in patterns of hydrological re-sponses will largely mirror shifts in overall precipitation patterns.Hydrological responses are, however, subject to complexfeedforward and feedback interactions in which any changes inprecipitation (∆P) on vegetated surfaces are modulated by thereductions that increased carbon dioxide levels (+∆CO2) haveon transpiration through increases in stomatal conductance, whileall this may, in turn, be counteracted by positive changes in tem-perature (+∆T), which would increase soil water evaporation (5).

In order to illustrate that changes in, say, runoff and net irri-gation demand display different spatial responses to the samechanges in overall precipitation, the climate change scenario gen-

erator SCENGEN (12), which facilitates the generation of glo-bal and regional climate change scenarios using results from se-lected general circulation models (GSMs), was applied to south-ern Africa. For a simulation of monthly ∆P and ∆T betweenpresent climate and that predicted for the middle of the next cen-tury, the Hadley Centre’s UKTR transient model (13) was se-lected to operate with the mid-range IS92a emission scenario(14) using the Intergovernmental Panel on Climate Change’s(IPCC) mid-range global temperature change of 2.5°C. Thismodel gave a warming of 1.7°C between the 1961–1990 base-line temperature and that for the decade of 2050. For spatial ana-lytical purposes the 5° latitude/longitude grid of ∆P and ∆T val-ues from SCENGEN was interpolated to a 1/4° grid by an in-verse distance weighting procedure (15), and the interpolated val-ues of monthly ∆P and ∆T were then used to perturb the cli-mate databases of the 1946 Quaternary Catchments coveringsouthern Africa. The ACRU model was then run for each Qua-ternary Catchment to produce values of various hydrological re-sponses.

Figure 11. Simulated differencesin mean annual rainfall, runoff andnet irrigation requirements oversouthern Africa for a “future”minus present climate, using theHadley Centre UKMO transientmodel in conjunction with theACRU model (15).

Figure 12. Comparison of seasonal streamflowsin the Bivane catchment from recorded dailyrainfalls, from downscaled daily rainfallsassuming correct rainfall forecast categoriesand those from actual forecast categories (16).


The three maps making up Figure 11clearly support the hypothesis that differ-ent hydrological responses to global cli-mate change do not simply mirrorchanges in overall precipitation patterns.The relatively large model prediction ofa decrease in MAP along the south andeast coasts (Fig. 11, top) does not showup in the map depicting changes in run-off (Fig. 11, middle), which changes mostin a block around 27°S and 31°E. The ap-parently anomalous increase in runoff isin contrast to an expected decrease and isthought to be due to the ACRU model’sovercompensating for the CO2 effects oftranspiration suppression. (This routine inthe ACRU model has subsequently beenrevised). The change in net water require-ments for supplementary irrigation ofmaize in summer (plant date 1 November;length of growing season 140 days) andwheat in winter (plant date 1 May; lengthof growing season 150 days) is highest inthe semiarid zone from 29° to 34°S and21° to 25°E (Fig. 11, bottom), i.e. in a to-tally different region to where changes inMAP and MAR occur. This illustratesclearly the different responses of compo-nents of the hydrological system in dif-ferent regions in an enhanced greenhouseclimate scenario.

Issue 8. Hydrological concerns indeveloping countries are focused moreon inter-seasonal scales than on decadalscales of climate change.To the man in the street in rural Africa,the potential impacts of the enhancedgreenhouse effect through the effectivedoubling of atmospheric CO2 concentra-tions by (say) the year 2050 are often anincomprehensible scientific issue of noimmediate concern. When, however, or-dinary farmers as well as water manag-ers start watching for Southern OscillationIndex trends and start talking of the on-set of the next “Hell” Niño, these are

Figure 13. Sensitivity of runoff (top) and net irrigation demand (bottom) to climate change insouthern Africa (15).

casts which were made were to have been used in the Bivanecatchment simulation, rather than assuming the rainfall forecaststo have been correct, the results are not yet as encouraging (Fig.12, bottom left).

The example above highlights the potential application of sea-sonal rainfall forecasts in managing water resources by obtain-ing, with some confidence, an idea of the anticipated streamflowsa season ahead.

Issue 9. In order to be proactive in regard to long-termclimate change, there is a need to identify hydrologicallysensitive areas.In identifying regions which are more sensitive or less sensitivethan others to climate change one examines the “elasticity” of aselected hydrological variable, i.e. the relative change of thatvariable, such as runoff or irrigation demand, to a change or com-bination of changes in a climatic forcing function (such as rain-fall or temperature or potential evaporation). Results from asouthern African spatial sensitivity study (15) are presented be-low. For an assumed uniform temperature increase of 2°C un-der x2CO2 conditions, precipitation was initially maintained at

“real” issues of the day, because food and water security are atstake to them. The correct forecasting of the onset, duration andintensity of the next El Niño, and converting this informationinto operational decision-making in water resources, then be-come scientific issues of relevance.

Two questions arise. First, if simple, but critical rainfall cat-egories, such as above-normal, normal or below-normal couldbe forecast a season ahead, how well could hydrologists utilizethis information? Second, how much would results improve forcorrect vs actual forecasts?

It has been shown for the Bivane catchment in KwaZulu-Na-tal province in South Africa (16) that if the categorical seasonalrainfall forecasts were correct, that by appropriate techniques ofdownscaling from seasonal categorical forecasts to daily rainfalls(the Seasonal Forecast Rainfall Builder, SFRB in Figure 12), amodel can be used successfully to forecast rainy seasonstreamflows (Fig. 12). However, extreme events—such as theheavy 600 mm 3-day rainfall and associated runoff coupled withcyclone Domoina in 1984—cannot yet be predicted a seasonahead. Furthermore, the skill in forecasting correctly the categoryof a season’s rainfall is still developing and if the actual fore-


References1. Turner, B.L., Skole, B., Sanderson, S., Fischer, G., Fresco, L. and Leemans, R. 1995.

Land-Use and Land-Cover Change: Science Research Plan. IGBP Report, 35. IGBP,Stockholm, Sweden, 132 pp.

2. COHS. 1991. Opportunities in the Hydrologic Sciences. National Research Council.Committee on Opportunities in the Hydrologic Sciences, Water Science and Technol-ogy Board: Commission on Geosciences, Environment and Resources. National Acad-emy Press, Washington DC, USA, 348 pp.

3. Nemec, J. 1995. General circulation models, climate change, scaling and hydrology.In: Kite, G. W. (ed). Time and the River. Water Resources Publications, HighlandsRanch, CO, USA, pp 317–356.

4. Schulze, R.E. 1995. Hydrology and Agrohydrology. Water Research Commission, Pre-toria, RSA, 552 pp.

5. Schulze, R.E. 1997. Impacts of global climate change in a hydrologically vulnerableregion: Challenges to South African hydrologists. Progr. Phys. Geogr. 21, 113–136.

6. Kienzle, S.W., Lorentz, S.A. and Schulze, R.E. 1997. Hydrology and Water Qualityof the Mgeni Catchment. Water Research Commission, Pretoria, RSA, 88 pp.

7. Schulze, R.E. 1997. South African Atlas of Agrohydrology and Climatology. Water Re-search Commission, Pretoria, RSA, 276 pp.

8. Acocks, J.P.H. 1988. Veld Types of South Africa. Botanical Survey of South Africa,Memoirs, 57. Botanical Research Institute, Pretoria, RSA, 146 pp.

9. Scott, D.F. and Schulze, R.E. 1992. The hydrological effects of a wildfire in a euca-lypt afforested catchment. South African Forestry J. 160, 67–78.

10. Schulze, R.E., Horan, M. and Perks, L. 1997. Water resources assessment in the Nadicatchment for community requirements at Ehlanzeni. ACRUcons Report, 22. Univer-sity of Natal, Department of Agricultural Engineering, Pietermaritzburg, RSA, 24 pp.

11. Schulze, R.E., Taylor, V., Matthews, G. and Hughes, G.O. 1997. Hydrological impactsof land use practices in the Pongola-Bivane catchment, Phase 2: Re-assessment of landuses in the Bivane catchment and hydrological impacts thereof. ACRUcons Report, 20.University of Natal, Department of Agricultural Engineering, Pietermaritzburg, RSA,22 pp.

12. Hulme, M., Jiang, T. and Wigley, T.L.M. 1996. SCENGEN, A Climate Change Sce-nario Generator: User Manual. University of East Anglia, Climatic Research Unit, Nor-wich, UK, 38 pp.

13. Murphy, J.M. and Mitchell, J.F.B. 1995. Transient response of the Hadley Centre cou-pled ocean-atmosphere model to increasing carbon dioxide. Part 2, Spatial and tempo-ral structure of the response. J. Climate 8, 57–80.

14. Legget, J., Pepper, W.J. and Swart, R.J. 1992. Emissions scenarios for the IPCC: Anupdate. In: Houghton, J.T., Callander, B.A. and Varney, S.K. (eds). Climate Change1992: The Supplementary Report to the IPCC Scientific Assessment. Cambridge Uni-versity Press, Cambridge, UK, 38 pp.

15. Lowe, K.L. 1997. Agrohydrological Sensitivity Analysis with Regard to Projected Cli-mate Change in Southern Africa. MSc dissertation. University of Natal, School of En-vironment and Development, Pietermaritzburg, RSA, 104 pp.

16. Lecler, N.L., Schulze, R.E. and Pike, A. 1996. Initial assessment of methodologies togenerate agricultural and hydrological forecasts from seasonal categorical rainfall fore-casts in South Africa. Proceedings: Workshop on Reducing Climate Related Vulner-ability in Southern Africa. NOAA, Office of Global Programs, Washington DC, USA,pp 113–130.

17. Acknowledgements. The research results reported in this paper have emanated fromprojects funded by the Water Research Commission, the Foundation for Research De-velopment and ESKOM, all from South Africa, as well as NOAA from the USA. Theirsupport is acknowledged gratefully. I should also like to express my thanks to colleaguesGregory Kiker, Bradford Howe, Mark Horan and Lucille Perks for assistance with ana-lytical/GIS work. All computations and GIS mapping were performed at the Comput-ing Centre for Water Research at the University of Natal, and the Centre is thankedfor making its facilities available.

Roland Schulze is a professor of hydrology in the School ofBioresources Engineering and Environmental Hydrology atthe University of Natal in Pietermaritzburg, South Africa.His research interests are in applied hydrological modelling,climate and land use change impacts and in integratedwater resources management. He is a member of theScientific Steering Committee of IGBP-BAHC.His address: School of Bioresources Engineering andEnvironmental Hydrology, University of Natal,Pietermaritzburg, P. Bag X01, 3209 Scottsville, South AfricaE-mail: [email protected]

“no change” from the present, and then perturbed by +10% andby –10% per day in the ACRU model for the climate databasesin southern Africa. Model outputs for the three assumed precipi-tation regimes (∆P of +10%, 0%, –10%) were then incorporatedinto a sensitivity equation such that

S = [(X+10 – X–10) / X0] / [P+10 – P–10) / P0]

in which S is the sensitivity index, X is the hydrological vari-able being assessed (e.g. runoff, or irrigation demand), P is theprecipitation forcing function and subscripts +10, 0 and -10 re-fer to possible percentage perturbations of precipitation. Thehigher the value of S, the greater the sensitivity, i.e. the greaterthe relative change of the variable to a unit change in precipita-tion; hence the greater the nonlinear response to precipitation.

Figure 13 (top) shows that for X = runoff, the sensitivity in-dex ranges from < 2 (moderately sensitive) to > 5 (highly sen-sitive). The map shows that it is not necessarily the present highrainfall or runoff areas (cf Fig. 4) that are more sensitive to rela-tive changes of precipitation. This indicates also that more com-plex interactions are at play in runoff generation, for example,those related to soil properties or to antecedent wetness condi-tions. In the sensitivity study, where X = net supplementary ir-rigation demand, results of which are shown in Figure 13 (bot-tom), two striking differences to the runoff sensitivity mapemerge. First, the highest sensitivity is, in this case, in the higherrainfall areas of the subcontinent (eastern and southern regions),where increased temperatures have a higher relative impact onsupplementary irrigation water demands under different rainfallregimes than in more arid areas, where there is so little rainfallin the first instance that a +10% or –10% rainfall change hasvirtually no impact on an already high irrigation water demand.Secondly, it is significant to note that the values of the irriga-tion sensitivity index are an order of magnitude lower than thoseof runoff responses.

CONCLUSIONSThis paper has addressed the issue of land use, hydrology, andsimulation modelling within a context of current climates as wellas of climate change. Examples have been selected from experi-ments and modelling exercises in southern Africa and they werechosen to cross a range of spatial and temporal scales. In thefinal analysis, the issues presented in this paper raise some ques-tions to the IGBP and its core programmes. These include thefollowing:How and when will one detect long-term climate change effectsin a region where high natural variability is a dominating fea-ture of both climate and hydrology?Already existing, and often complex, land use patterns have sig-nificant impacts on hydrological responses. Should such stud-ies not be pursued with the same vigor as the anticipated shiftsmodelled in natural vegetation belts under conditions of climatechange?Is the LUCC initiative addressing land use change and its im-pacts at appropriate scales for “real life” decisions to be madein, say, an African context?Is BAHC, in its endeavors, addressing “real” issues of hydrol-ogy of, say, Africa and the modelling thereof, for actual deci-sions to be made for the welfare of developing populations?Should the climate change paradigm shift from researchingdecadal scale prediction to seasonal scale forecasting and its op-erational impacts, a shift which is already evident, not be fur-ther encouraged?In summary, are we active scientists in the IGBP programs fo-cusing enough on actual problems of the hydrologically relatedenvironment on a continent with real, day-to-day problems ofexistence?

modelling hydrological responses to land use and climate change: a southern african perspective

Documents