CHIRPS 2.0 prelim (sub 1)

Tanzania data and trends

Comparison to CHIRPS v1.91/27/2015

Laura Harrison

Overview 1: NDVI and rainfall trends

Main differences• Parts of central TZ are

wetter now on average• Acceptable• But the clear 30-year drying

trend that matched with the 30-year NDVI declines is now gone

Time series comparisons• Show that CHIRPS 2.0 prelim

seems fine• In some areas more strongly

correlated with NDVI than before

• Questionable areas– Shinyanga-Tabora central area.

2 stations here. Are they ok?– Area of greening trend (see

Overview 2: Seasonal Trends)

Rainfall isohyets (Oct-May total)Base layer: NDVI-3g (Nov-Oct mean)

CHIRPS v1.9 CHIRPS v2.0p Wetter climate

Topography

Slope of linear trend 1982-2010Oct-May rainfall total

CHIRPS v1.9 CHIRPS v2.0p

Weaker drying trend:• Central TZ• North-central TZ

Using z-scores

Slope of linear trend 1982-2010Oct-May rainfall total and Nov-Oct NDVI mean

CHIRPS v1.9 NDVI-3gPreviously in CHIRPS

(x 2.5 fix)

Box 1

Box 2

Box 3

Box 4

Using z-scores

Slope of linear trend 1982-2010Oct-May rainfall total and Nov-Oct NDVI mean

CHIRPS v2.0p NDVI-3gCurrently in CHIRPS

Box 1

Box 2

Box 3

Box 4

Using z-scores

Rainfall and NDVI time seriesBox 1: Shinyanga-Tabora west

CHIRPS v1.9 CHIRPS v2.0

NDVI improvesRainfall does not

NDVI improvesRainfall improves

Stronger correlation

No rainfall trend (1982-2010)Rainfall trend is significant and matches NDVI trend (1982-2010)

Reality check: OK

Using z-scores

Rainfall and NDVI time seriesBox 2: Shinyanga-Tabora central

CHIRPS v1.9 CHIRPS v2.0

Weaker correlation

No rainfall trend (1982-2010)Rainfall trend is significant and matches NDVI trend (1982-2010)

Extremes are mostly wetter, but not always.• Wetter wet years: 1997-98, 2006-07• Wetter dry years: 2007-08, 2010-11• Drier dry years: 1983-84, 1999-00

Wet yearsWet years

Dry years Dry years

Reality check: ?

There are 2 stationsin this area

Using z-scores

Rainfall and NDVI time seriesBox 3: Singida

CHIRPS v1.9 CHIRPS v2.0

Same correlation

Rainfall trend not significantRainfall trend is significant (1982-2010)

Minimal difference

Reality check: OK

Using z-scores

Rainfall and NDVI time seriesBox 4: Arusha

CHIRPS v1.9 CHIRPS v2.0

Stronger correlation

Reality check: OK

Using z-scores

Overview 2: Seasonal trends

Main differences• Oct-Feb trends

– Weaker drying trends– Significant drying trends gone in

Shinyanga (TZ), Congo, Rwanda– Significant drying trend still in southern TZ

• Mar-May trends– Weaker drying trends– Arusha area shows only a weak Long Rains

drying trend– New wetting trend (significant) in

northern TZ– Significant drying trends still in Tanga &

Morogoro

• Oct-May total trends (revisited)– Significant drying trends gone

in central and north-central TZ Congo, Rwanda

– Significant drying trends still in southern TZ

– New significant drying trends in east TZ

– Stronger, larger drying trends in Kenya

Slope of Sen trend 1982-2010Oct-May rainfall total

CHIRPS v1.9 CHIRPS v2.0pPercent change per decade Stippling shows significance at p < 0.05

Overall: Weaker drying trends in central and north-central TZ

Stronger Kenya trends

Previously an area with significant drying trend

Slope of Sen trend 1982-2010Oct-Feb rainfall total

CHIRPS v1.9 CHIRPS v2.0pPercent change per decade Stippling shows significance at p < 0.05

Previously an area with significant drying trendOverall: Weaker Oct-Feb trends; only

southern TZ is significant in CHIRPS v2.0p

Slope of Sen trend 1982-2010Mar-May rainfall total

CHIRPS v1.9 CHIRPS v2.0pPercent change per decade Stippling shows significance at p < 0.05

Previously an area with significant drying trendLoss of Long Rains drying trend

New wetting trends