A systematic approach to improve the planning, design and operation of surface water management systems : a case study

From an agricultural point of view the most desired surface water level in areas with a shallow groundwater table is low during winter and high during the growing season. Waterboards in the Netherlands try to fulfil this demand by applying different surface water levels in winter and summer.Because weather conditions vary considerably from year to year, the most desired open water level should be varied too.'The manipulation of weirs by the Auterboards is mainly based on practical experience and is riot much different from year to year. To obtain better founded rules for surface water manipulation, a study was carried out in a cut-over peat region of about 8000 ha.For the surface water, the groundwater and the unsaturated soil water system in this area a simulation model was constructed that links these systems mutually and offers the possibility of computing consequences of manipulating open water levels for the water use by crops. The model was calibrated with hydrological data collected in the area. operational rules for setting weirs and inlet structures were established by comparing the effects of a number of possible rules on depth of groundwater tables and consequent water uptake by the crops for a number of meteorological years. With the operational rules that gave the largest effects m water use by the crops management strategies involving water conservation and additional water supply capacities for sub-irrigation were simulated with the model. The additional transpiration by the crops caused by these strategies were converted to extra yields so that an economical analysis of these strategies became possible. With the aid of the results of this analysis also a demand function for water for the study area was derived. Finally the possible application of the demand function in the water management policy at provincial level was outlined.The results show that the proposed model offers good possibilities to forecast effects of water management strategies. It can be used by a waterboard to decide upon changes in open water levels that are required in the course of the season because of weather conditions. The weakest point in the model seems to be the lack of knowledge about the possible negative effects of waterlogging on transpiration by crops.For the conditions prevailing in the study area, water conservation is economically very attractive. Additional water supply to the area for sub-irrigation has a relatively low efficiency, but the economical analysis shows that investments in this strategy may pay off too.