Transcript Sahysmod Spatial Agro-Hydro

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 separately
2. 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
Surface level
Bottom level
Water level
Elevation (m)
250
200
150
100
50
0
200
400
600
700
800
900 1000 1100 1300 1400
Distance (m)
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.