Sustainable use of groundwater for irrigation: A numerical analysis of the subsoil water fluxes

The food-producing regions of the world increasingly rely on irrigation from groundwater resources. Further increases of groundwater use can adversely affect the sustainability of irrigated agriculture and put food security at risk. Sustainability of irrigation at field scale with groundwater is obtained if groundwater recharge is in equilibrium with tubewell extractions and capillary rise. Traditional information on phreatic surface behaviour does not explain the processes causing a phreatic surface to decline or incline. In this study, the physically based numerical model Soil-Water-Atmosphere-Plant (SWAP) was applied to compute soil moisture content and vertical soil water fluxes in the unsaturated zone for the cotton-wheat and rice-wheat cropping system of Punjab, Pakistan. SWAP has been calibrated and verified with in situ measurements of soil moisture content and evapotranspiration fluxes measured by means of the Bowen ratio surface energy balance technique. Accurate data of the soil hydraulic properties are critical for the calibration of the soil moisture distribution. With knowledge of the van Genuchten-Mualem parameters available, SWAP could be applied to assess recharge and capillary rise for most field conditions, including basin irrigation. The results under Pakistani conditions show that deep percolation cannot always be estimated from root zone water balances. An annual recharge of 23.3 cm was computed for the cotton-wheat area. Sustainability of irrigation with groundwater is obtained if a reduction in irrigation with groundwater by 36% is obtained. An annual recharge of 38.9 cm is estimated in rice-wheat systems, and a reduction of 62% in groundwater extraction is required to reach sustainability of groundwater use at field scale. Such information cannot be obtained from classical phreatic surface fluctuation data, and unsaturated zone modelling therefore provides additional insights for groundwater policy making