SAHYSMOD

Spatial Agro-Hydro-Salinity Model

INTRO

Sahysmod is a computer program for the prediction of the salinity of soil moisture, ground water and drainage water, the depth of the water table, and the drain discharge in irrigated agricultural lands, using different (geo)hydrologic conditions, varying water management options, including the use of ground water for irrigation, and several cropping rotation schedules, whereby the spatial variations are accounted for through a network of polygons.

Sahysmod working group

Sahysmod combines the agro-hydro-salinity model Sahysmod (Oosterbaan 1998) and the nodal (polygonal) ground water model SGMP (Boonstra and de Ridder 1981). The combination was made by K.V.G.K. Rao with guidance from J. Boonstra and R.J. Oosterbaan, the user menu by H. Ramnandanlal, R.A.L. Kselik and R. J. Oosterbaan to facilitate the management of input and output data. These five persons formed the Sahysmod working group of ILRI with Oosterbaan as co-ordinator and editor. He also rebuilt the program to reduce the computer memory requirements and to increase the maximum number of polygons.

Polygons

The principle of Sahysmod is a network of nodal points (nodes) with coordinates. Then, polygons are made around the nodes using the principle of Thiessen.

In each polygon:1. Saltmod is applied separately2. Groundwater flow is calculated from water

levels in neighbouring polygons and hydraulic conductivity between the polygons using small time steps.

Example 1: a large nodal network, case study Garmsar

Example 2: simple nodal network, case study Icmald

Case study Icmald

• In case study Icmald there is only one line of internal polgons surrounded by external polygons for boundary conditions.

• The hydraulic conductivity between internal and external polygons is made zero so that the flow can not spread out, it is only in one direction.

• This pattern is useful to calculate conditions in a cross-section over a valley from upland to the bottom land and river

Cross-section Icmald

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Distance (m)

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Surface level Bottom level Water level

Details Icmald

• In the centre of the area, from left to right, there is a leaking irrigation canal.

• In the downstream part of the area there are waterlogging and salinity problems

• First we simulate the effect of canal lining.

• Secondly we simulate the effect of interceptor drain along the canal.

• The results are shown in the next table

Example of result case study Icmald

Depth of water table (Dw, m) in Year 3, Season 1 ----------------------------------------------------------- Polygon Reference Canal Interceptor no. situation lining drain ----------------------------------------------------------- 1 9.14 9.39 9.36 2 6.89 7.22 7.16 3 4.69 5.13 5.03 4 3.61 4.11 3.98 5 2.54 3.10 2.94 6 1.62 2.11 2.05 7 0.71 1.13 1.09 8 0.27 0.62 0.60 9 0.64 0.94 0.93 10 0.35 0.63 0.63 ------------------------------------------------------------

Results

• We can see that canal lining and interception drainage have a small effect in the lower part, because the infiltration losses from the canal are small compared to the deep percolation losses from field irrigation in the upper area.

• In the lower part there is little irrigation due to water logging and salinity. If we increase the irrigation for reclamation and cropping, the water table will become very shallow again.

• Canal lining or interception drainage are not sufficient to cure the problem.

• If enough irrigation water is available, the lower part can be reclaimed using normal drainage system or wells. This can also be simulated.

The upper external polygon has very salty ground water (50 dS/m, like sea water). We can analyze its displacement.

3 Salinity (Cqf, dS/m) of the aquifer ------------------------------------------------------ Polygon -------------------------------- Year Season 1 2 3 ------------------------------------------------------ 0 0 1.0 1.0 1.0 1 1 4.4 1.1 1.0 2 8.0 1.5 1.0 2 1 10.9 2.1 1.1 2 13.7 2.9 1.3 3 1 16.2 3.8 1.5 2 18.5 4.8 1.8 -------------------------------------------------------

Example 3, Hansi Farm, India

In Hansi Farm, the following polygonal network was used

Hansi case study

• In Hansi Farm, there is natural drainage through the aquifer to the neighbouring areas because the water level in Hansi is higher.

• The neighbouring areas recieve upward seepage of groundwater and are in danger of salinization.

• Sahysmod was used to determine drain discharge at different drain depths of possible drainage systems in Hansi Farm.

Results from Hansi case study

• It was found that deeper drains discharge more water because the water level is lower and underground outflow is less.

• When the drainage level is deeper than 2 m, in some polygons the natural underground outflow changes into underground inflow, causing upward seepage of ground water, so that the drain discharge is even more.

• Some data are given in the next slide

Some data from Hansi results

• In polygon 1 the water table drops from 3.0 m depth to 3.2 m depth even though the drain discharge is always zero. This shows that polygon 1 does not need drainage, but some water from polygon 1 goes to the drains of neighbouring polygons. by drainage other polygons.

• In polygon 8, the present groundwater outflow is 2.8 m/year. This indicates excessive irrigation.

• In polygon 12 the drain discharge without drainage system is only 0.46 m/year. With drainage level at 2 m. depth it is 4.3 m/year. Hence, deep drainage attracts much water from neighbouring nodes.

• Hence, the drain discharge is influenced by the drain depth.

SUMMARY

• Sahysmod can be used for many different situations and purposes.

• Only two examples were given and only a few aspects of these examples were discussed.

• More information on these examples can be found in the manual that can be downloaded from website www.waterlog.info under Articles/Manuals.

• Also the program itself can be downloaded freely from this website under Software

Notes• The outcomes can be checked by hand, even

though the calculations are tedious.

• The output of Sahysmod can be saved in spreadsheet files. These can be used for further analysis in:

1. Spreadsheets (e.g. Excel)

2. GIS, Surfer, Winsurf, etc. (for mapping)

• A GIS example follows

Initial values 1995, calculated and measured 1996

Calculated and measured 1990

• This ends the power point presentation.