Distributed agro-hydrological modeling of an irrigation system in western Turkey

A clear understanding of all the components of the water balance is essential to analyze possible measures of water savings in irrigated agriculture. However, most components of the water balance are not easily measurable either in terms of the required time interval or the complexity of the processes. For an irrigated area in the Western part of Turkey, the physically based one-dimensional agro-hydrological model SWAP for water transport and crop growth was applied in a distributed manner to reveal all the terms of the water balance. A combination of point data and distributed areal data was used as input for the model. The emphasis was put on the conversion from available data to required data. Simulations were carried out for the period 1985–1996 and detailed analyses were performed for two successive years, a pre-drought year, 1988, and a dry year, 1989. Irrigation deliveries were reduced substantially in 1989, resulting in a modified water balance. Lateral fluxes to drains were reduced, bottom fluxes were changed from groundwater recharge to capillary rise, evapotranspiration was lower, and, most importantly, relative yield was reduced. All these changes were obtained for specific sites by combining existing soil data with cropping patterns. It was concluded that the use of the SWAP model in a distributed way is a useful tool to analyze all the components of the water balance for a whole irrigation system.