This could be best explained, as the watertable along the drainage canals has found shallower relatively due to low location. The irrigation canals are constructed on relatively high locations comparing to the drainage system to utilize the natural gradient effectively. The occurrence pattern of salt-affected soils through unsupervised classification method was grouped into two main classes i.e. slightly to normal saline and visible salt-affected soils. The existence percentage of salt-affected soils was found to be 7.5 % for the mapped area of S-1 unit. Map of 'mean' depth of watertable shows the extent of waterlogging (area _ 200cm) in the area. The yearly average watertable analysis indicates the lowering trends during 1990-93 from 24.5 % to 22. %. However, it started increasing tremendously by 39 % in 1996. This situation is quite alarming when comparing the change of watertable for the 7 years period gape. Low with-drawl from the sumps, low natural drainage to the aquifer, deterioration of many vertical drainage system (tube-wells), choking of various sub-surface drainage lines by debris like weed growth and silting have appreciably contributed to the rising of watertable (WAPDA, 1997). All other sources of recharge and discharge of groundwater reservoir are, more or less, the same except the unpredictable rainfall pattern but it hardly cause any big change in the hydraulic balance of the area. Many farmers were reported for re-use of poor quality drainable surplus to supplement irrigation needs. Most of the farmers complained water shortage in the socio-economic survey (WAPDA/SMO, WM&E, 1993-94) for the whole area, especially, big percentage for the tail-end users. Inequity water distribution were also reported among the shareholders of a watercourse at the head, middle and tail reach.
Agro-Productivity in Relation to Hydro-Salinity
The results of the vegetation index (NDVI) were analyzed along the irrigation and drainage channels (the area under easy risk of hydro-salinity) indicated rather scarcity of vegetation as a whole (Figure 6). Relatively healthy growth of the green bio-mass were observed along the canals than that along the drains. Easily visible high NDVI were noted for a private agricultural model farm (Sehgal model farm) near the FDP area for all periodic NDVI images, except fellow lands. It is due the fact that the water management is demand oriented there (3-times more water allocation
Figure 6. Vegetation statusthrough NDVIwithin 1000m buffered zone along the
irrigation and drainagechannels.
i.e. 4.7mm/d than the average of FDP area as 1.54mm/d), thus more flexible with sufficient water supply. The design irrigation allocation (1.8-2.1mm/d) for the area is half of average crop water requirement (JIID, 1997). These results clearly show the possibility of resolving the remaining salinity problem out of area. The NPP (Net Primary Productivity) for natural vegetation corresponding to the annual precipitation range of about 300mm was too small and quite sensitive to changes in the rainfall pattern (JIID, 1997). As NPP is closely related to the
agricultural productivity in rainfed farming, it is certain that ample irrigation supplies are necessary to obtain sustainable agriculture in the area.
Conclusion
Composite image and unsupervised classified image shows that most of waterlogging and salinity problems are located along the drainage canals. The existence percentage of salt-affected soils was found to be 7.5 % for the mapped area of S-1 unit. Th situation of waterlogging is quite alarming when comparing the change of watertable for the 7 years period with the lowering trends from 24.5% to 22% (1990-93) and clear increase by 39% in 1996. Evidence is presented that the existing drainage system along with use of groundwater for irrigation reduces the extent of waterlogged soils. However, the reuse of poor quality water to supplement irrigation supplies by the downstream farmers and the failure of a few drainage sumps are likely to disturb the water balance resulting into increased risk of waterlogging and salinity in the area. The vegetation growth analyzed through NDVI tends to be lower along drainage and higher along the irrigation canals due to the inequity water distribution and locational disadvantage of the drains and vice versa. High NDVI values for almost all over the model farm near FDP project area with demand oriented water supply thus gives the solution to resolve the remaining salinity problem. Lastly, it is concluded that the water shortage is one of the fundamental problem in the area to reclaim the soils for sustainable productive lands.
References
Casas, S., 1995. Salinity assessment based on combined use of remote sensing and GIS. Use of Remote Sensing Techniques in Irrigation and Drainage, pp. 141-150, FAO, Rome, Italy. JIID, 1997. Study on Establishment of a New Monitoring System for Land and Water Resources Management in Arid Area. (JIID), Main Report, 254 p.
SMO (M&E Directorate, WAPDA), 1994. Annual report. M&E, FDP Project. Vol.I. Text Pub. 237. Srivastava, A.,Tripathi,N.K.,and Gokhale, K.V.G.K.,1997. Mapping groundwater salinity using IRS-1B LISS II data and GIS techniques. Int. J. Remote Sensing, vol.18, NO.13,pp.2853-2862. Tripathi, N.K., Rai, B.K., and Dwivedi, P., 1997. Spatial modeling of soil alkalinity in GIS environment using IRS data.18 th Asian conference on remote sensing, Kualalampur, pp.A.8.1-A.8.6.
WAPDA, 1997. Monitoring and Evaluation Fourth Drainage Project, WAPDA, Lahore, Pakistan.
