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Food production, water availability and energy production are important ecosystem services of the Upper Tana basin (Kenya) and they decline due to upstream erosion affecting downstream water users. The effect of 11 soil conservation measures on soil erosion and the three ecosystem services was estimated by a modelling approach to assess agro-ecological processes and benefit/cost relations. Soil water available for evaporation and transpiration (‘green water’) functioned as a unifying concept to express the effects of erosion and the impacts of soil and water conservation measures that result in: (1) increased water availability for crops; (2) increased fluxes towards aquifers, thereby increasing water supply and regulating streamflow, and (3) a reduction of erosion and siltation of reservoirs used for hydroelectricity. Modelling indicated that the three ecosystem services could be improved, as compared with the base level, by up to 20% by introducing appropriate conservation measures with benefit/cost relations of around 7. However, farmers were unable to make the necessary investments and much effort and many institutional studies were needed to achieve progress towards implementation by initiating the Green Water Credits (GWC) programme intended to arrange payments by downstream businesses to upstream farmers. A timeline analysis is presented to illustrate the slow, but persistent, development of transdisciplinary activities as a function of time using connected value developmentas a guiding principle.

An increase in the amount of water used for agriculture has resulted in the need for water savings and measurements in the agricultural sector. With the increasing competition for water, not only should irrigation water be considered, but also an approach encompassing the total water balance is necessary. A clear understanding of all the components of the water balance is essential so as to analyse possible water saving measures. In this study, a one-dimensional physically based soil-water-atmosphere-plant model was applied in a distributed way in order to analyse all the water balance terms of an irrigated area. Methodology and data requirements are described. The results were concentrated on a comparison between a pre-drought and a drought year for Salihli Right Bank Irrigation area in the Gediz Basin, Western Turkey. The whole study area was divided into sub-areas as Land Use Systems, then this simulation model was applied to include the essential parts of the water balance such as soil evaporation, crop transpiration, groundwater recharge, capillary rise, and crop growth as a function of soil water dynamics. It was concluded that the use of existing data in combination with a physically based simulation model is a useful tool to analyse all the components of the water balance of irrigation systems.

Abstract:  Worsening water scarcity will increase pressure to use water more productively. In the classical view of irrigation research, some important aspects are often ignored: the total water balance approach, productivity of water, food security, and irrigation-system level analyses. These four approaches were evaluated using a detailed agro-hydrological model applied to an irrigation system in western Turkey. Emphasis was placed on the two dominant crops in the area: cotton and grapes. According to the classical point of view, the only result would be to irrigate the cotton with 1000 mm and the grapes with 800 mm. From the water productivity point of view, however, the water productivity of grapes appeared to be maximal without any irrigation; while for the cotton, irrigation at 600 mm maximizes water productivity. To minimize risks and increase yield stability, grapes perform better than cotton. Finally, from the irrigation system point of view, decisions can be made about the desirable cropping pattern and the distribution of water between crops. With limited amounts of water available for irrigation, a cropping pattern consisting mainly of grapes is desired; while with higher water availability, a mixture of cotton and grapes is preferable. The methods presented provide a clear methodology with which to achieve the most productive use of water.