Keywords: water harvesting tank, simulation model, watershed, semi-arid region.

This paper shows the utility of a simulation model for the design of water harvesting tanks in agricultural watersheds. The concept of tank type based on the relative positions of catchment and command area has been introduced into the model. The detail discussion of tank types can be found in [1]. The model was developed to include and study the effect of different tank strategies on the volume of the irrigation water made available. The model evaluates the different combinations of the number of tanks, their locations, and sizes in respect of volume of irrigation water generated in the watershed. The most important feature of the study is that the model recognises the different nature of the catchment and the command area of tank. All the supply and demand parameters affecting the tank capacity are considered in the analysis. The paper also discusses the results of a watershed case study for a semi-arid region of India. The results of the case study indicate that fewer tanks with more capacity generate more irrigation volume than a greater number of tanks with less capacity. Tank size does not show any correlation with rainfall or inflow.

Input to the model comprises climatic data, watershed data and irrigation strategy. With the input of number of possible locations, the model generates a number of tank strategies based on the combinations of tank numbers, locations and their types. Simulation of field water balance, tank water balance and groundwater balance is carried out daily for each tank strategy and an estimate of the volume of irrigation water made available.

Simulation starts from 1st June of each year and the storage in the tank at the start of the simulation is considered zero. Rain over the tank surface area, runoff from the catchment and outflow from the upstream tanks are the inflow to the candidate tank. Outflow occurs when inflow exceeds the available capacity of the tank. The storage is then subjected to evaporation and seepage losses. Evaporation from tank is computed by the Penman method [2] method and seepage is considered depending on the type of soil. Irrigation time and volume is activated as per the user defined depletion and volume criteria. Runoff is computed by the SCS-CN method with the soil moisture accounting procedure as suggested in [3]. The water surface area and the wetted area of tank are updated daily to determine evaporation and seepage losses through the tank. The dimensions of the tank are optimised using the Lagrange method. To start the simulation, the initial tank capacity is required. This is computed with the help of some empirical value of design runoff depth which is user defined. Note that this is used to set off initial tank capacity only. For each tank strategy the tank size is determined after the simulation for the entire year is run. This computed outflow is compared with target outflow (this is user defined). If the criterion is not met, tank size is increased (or decreased) and simulation run again for the year. Thus the model arrives at a particular tank size for each tank strategy for each year after satisfying the irrigation and outflow criteria. The model can be run for a number of years for which climatic data is available.

The output of the model is number of tank strategies, number of tanks, location and type of each tank and the irrigation schedule for each crop grown on each field.

The case study results suggest construction of only one CCS type tank of capacity 9323 m³, at the outlet of the watershed (Location 1) to generate a maximum irrigation volume of 9994 m³ for a watershed of 11.5 ha. If it is not possible to locate the tank at the outlet then it may constructed at the middle of the watershed (Location 2) considering it as a CCM type to give the next maximum irrigation volume. When three tanks are to be constructed they should be constructed at locations 1,2 and 3: all of the CCS type. The resulting irrigation volume is 8768 m³. The model is thus capable of not only giving the number and locations of the tanks but also the type of tank as defined in the paper.

1

M.G. Shinde, I.K. Smout, S.D. Gorantiwar, "Design and performance indicators for water harvesting irrigation tanks in India", 30th WEDC International Conference, Vientiane, Lao PDR, 2004.

2

H..L. Penman, "Natural evaporation from open water, bare soil and grass", Proceedings of Royal Society, London, A193,120-146, 1948. doi:10.1098/rspa.1948.0037

3

A.N. Sharpley, J.R. Williams (Eds.), EPIC- Erosion/Productivity Impact Calculator: (1) Model Documentation, 235, 1990.

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