CONJUNCTIVE USE OF SURFACE AND GROUNDWATER TO IMPROVE FOOD PRODUCTIVITY IN A RESTRICTED AREA
A GROUNDWATER BALANCE STUDY TODEVELOP A TECHNIQUE TO IMPROVE THEGROUNDWATER SYSTEM IN A RESTRICTED AREA Research for the Degree of Doctor of Philosophy of Saravanamuttu Subramaniam SIVAKUMAR Supervised by Prof D.C.H.Senarath
Sequence of Presentation Problem Statement of Water Resources Thesis Statement Object of the Research Methodology of the Research Summary of Operational Research Summary of Economic Analysis of the Operational Research Summary of the Research Finding Conclusions Generalization Limitations Recommendations for Future Study
Problem Statement of Water Resource Economically feasible water storage sites are limited Unplanned utilization of various water resources by various stake holders Difficulty in analytical solution due to non homogeneous and anisotropic nature of groundwater resource
Thesis Statement“It is observed that at present the water that isavailable is not utilized effectively to achievemaximum productivity in terms of foodproduction”
Objective of the Research The objective of this research is a complete water balance study in a restricted catchment area incorporating few medium irrigation schemes, several minor Irrigation schemes and large number of dug wells to illustrate:1. The possibility of developing and using a model to represent all the relevant variables connected with the movement and utilization of surface and groundwater2. The possibility of using the above model to study the viability of conserving surface water by storage as groundwater by reducing the extent of cultivation using surface water and increasing the extent of cultivation using groundwater to achieve maximum crop yield3. The economic viability of achieving maximum crop yield as in (2)4. The possibility of creating an artificial aquifer boundary to optimize the effectiveness of groundwater use to achieve maximum crop yield5. The economic viability of the creation of artificial boundary in terms of productivity6. The possibility of combining both (2) and (4) for the increased crop production7. The economic viability of achieving maximum crop yield as in (6)
Methodology of the Research Concept General relationship between crop yield and water applied to the crop shows a trend to increase linearly up to about 50% of the full irrigation and then going in a convex curvature to the maximum yield and then reduce the yield with increase in applied water Farmers whose sole objective is to get maximum net Income, tends to irrigate their crop by spending minimum cost for their irrigation water to get maximum productivity for their crop“Hence the main methodology adopted in this researchregarding the optimum crop yield to maximize productivity interms of food production is economizing the cost of theirrigation water and increasing the extent of cultivation per unitof irrigation water”
Methodology of the Research Model Formulation A regional aquifer simulation model using integrated finite difference technique was formulated in spread sheet for a polygonal net work of a restricted catchments in Vavuniya with forty one observation wells This aquifer was divided into forty one polygons by connecting the perpendicular bisectors of adjoining observation wells. Six year seasonal water levels and one year monthly water levels, tank storage, field issues and total withdrawal from agro and domestic wells for each polygon were taken for the water balance of each polygon The model was calibrated by error optimization method using historic seasonal data. In each of the error optimization model four variables for polygonal inputs, one variable for that particular polygonal specific yield and five to seven variables for transmissibility for every polygonal connection were formulated with constrains.
Methodology of the Research Operational Research A study was carried out to find out an operational policy for conserving surface water by storage as groundwater by reducing the extent of cultivation using surface water and increasing the extent of cultivation using groundwater to achieve maximum crop yield under minor and medium irrigation schemes together with creation of an artificial boundary to lift the water table up By changing every polygonal input (i.e. to change the operational policy of minor and medium irrigation schemes to reduce the extent of cultivation using surface water and increase the extent of cultivation using groundwater) the water levels in each polygon were analyzed By changing the first interior boundary lateral flow to 50% in steps from its original value by assuming an artificial boundary, the variation of water levels in each polygon were analyzed By adopting combination of the above two strategies in different possible combinations, the variation of water levels in each polygon were analyzed The economic implication of the above three outcomes based on “Reducing the extent of cultivation will appear to be a loss to the Gross Domestic Product and lead to reduction in the Gross National Product too. But the gain in water table will reduce the cost of energy by way of fuel and electricity for the pumping of water for the cultivation, domestic and industrial water use. This will indirectly contribute to GDP and GNP positively”
Summary of Operational Research Changing the operational policy of minor and medium irrigation schemes by forgoing cultivation by 25% to 35% to conserve surface water by storage as groundwater is giving water table gain in almost all nodes except nodes 37and 38 by 1.75 ft to 3.0 ft during discharging season and by 2.5 ft to 3.75 ft during recharging season. This is a reduction of almost 45% to 65% of water table loss in between two consecutive seasons in 80% of the area of the catchments under study Creating artificial aquifer boundary to optimize the effectiveness of groundwater in an elevated water table by peripheral boundary treatment to cause reduction of permeability by 35% to 45% is giving water table raise of nodes closer to treated boundary by 1.5 ft to 2.75 ft during recharging season. Combining peripheral reduction in permeability by 35% to 45% and forgoing cultivation of minor and medium irrigation scheme by 45% to 55% result an average gain of water table during discharging season (June – Sept) 3.0 to 4.75 ft excluding node 37 and 38. The same trend is observed in recharging season to a lesser degree. This is a reduction of almost 60% to 70% of water table loss in between two consecutive seasons in 95% of the area of the catchments under study.
Summary of Economic Analysis of the Operational Research The alternative policy on changing the operational policy of minor and medium irrigation schemes by forgoing cultivation by 25% to 35% gave the benefit cost ratio based on present worth greater than unity with considerable rise in water table. The rise in water table occurred almost above 80% of the observation wells. The rise in water table was around 45% to 65% of the loss in water table between two consecutive seasons The boundary treatment showed positive results The combination of the above two alternatives yielded further improvement that, at any time water table will recover 60% to 70% of loss in between two consecutive seasons in 95% of the catchments under study. This implies that the boundary treatment combined with changing the operational policy of minor and medium irrigation schemes by forgoing a part of the cultivation is an economically feasible policy alternative.
Summary of the Research Finding“A change in operational policy of minor / mediumirrigation schemes by forgoing one third of thecultivation under minor / medium irrigation schemesor keeping one fourth of the storage of minor / mediumirrigation schemes at any time will recover an averageof 45% to 65% of the loss of water table in anyconsecutive seasons in almost 80% to 90% of thecatchments area under consideration”
Conclusions Minor / medium irrigation schemes conserve surface run off and covey most part of it to recharge groundwater and as such serves as a recharge shed for the wells situated in the zone of influence. It is an insurance against water scarcity, as the yield increases considerably for every unit of rainfall. The minor / medium irrigation schemes prevent soil erosion and depletion of soil fertility. In the context of impending water deficiency looming large, construction of minor / medium irrigation schemes will be a dependable infrastructure in the development of water potential in any catchments. Acknowledgement of the remarkable role played by the minor / medium irrigation schemes on replenishment of groundwater and its spread over a large area would be a great asset in planning and execution of settlement and crop production projects“Forgoing certain percentage of cultivation will be a loss to Gross DomesticProduct and lead to loss in Gross National Product also. The gain in watertable will reduce the cost of energy by way of fuel and electricity. This willindirectly contribute to GDP and to GNP and also reduce the cost ofirrigation water and in turn increase the extent of cultivation per unit ofirrigation water. This will increase the crop yield per unit of irrigation waterand to increase economic productivity in terms of food production”
GeneralizationThe model formulated for this research was applied to a selected catchment inVavuniya (a shallow weathered and rarely fractured rock with thin soil mantle)which covers 6 medium Irrigation schemes, 40 minor irrigation schemes andaround 2000 shallow wells within 71.5 sq.miles of area“The rationale behind the selection of this study area is that theaquifer of this region very well reflects the typical groundwaterproblems of unconfined aquifers in shallow weathered andrarely fractured rock with thin soil mantle. Study of this natureof problems is the prime intention of this research. Hence thefindings pertaining to this restricted catchment can begeneralized to any type of restricted catchment with certainlimitations”
LimitationsAn aquifer simulation model in Integrated Finite Difference Method used for thisresearch is applicable to any type of polygonal network with the following limitations Aquifer is a two-dimensional flow system Only one aquifer system is modeled Aquifer is bounded at the bottom by an impermeable layer There are no major irrigation schemes within the catchment above the aquifer
Recommendation for Future Study“Future study on policy alternatives towardsconjunctive use water management policy in anynonrestricted multilayered aquifer system withmajor irrigation schemes, will be very useful inmacro development of water resource indeveloping country like Srilanka”