Abstract
In large parts of the Netherlands, agricultural potential is largely determined by water table depth. In turn, the water table depth is controlled by the surface water levels of a network of drainage ditches. Consequently, the gross return from agricultural production can be maximised by manipulating the surface water levels
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in these ditches. The traditional mode of surface water control consists of a weir with a fixed crest level at the outlet of a control area or catchment. A recent development is the use of groundwater dependent surface water control: the crest level of an adjustable weir is set according to measurements of the water table depth. For both modes of surface water control the crest levels of the controlling weir are adapted to the surface elevation at some reference location. The gross return from agricultural production thus depends on both the mode of surface water control and the choice of the reference location. A method is developed to estimate the performance of a mode of surface water control when adapted to a given reference location. Performance is measured in terms of the estimated average gross return of a control area (in Dutch guilders per ha per year). The method combines a soil water model with detailed surface elevation data from laser scanning and accounts for the unknown spatial variation of soil-physical and hydrological properties by means of a Monte Carlo approach. Although it is developed for the specific purpose of evaluating modes of surface water control, the method presented here is more generic. It is in fact a general procedure that uses densely sampled auxiliary information for mapping the results of expensive model calculations and the associated uncertainty. The method is applied in a control area in the north of the Netherlands. It is used to compare the performance of a traditional mode of surface water control with a groundwater dependent mode of surface water control and to estimate the best reference location for both modes. It is found that the traditional mode of surface water control is best adapted to a reference location from the higher parts of the control area, while the groundwater dependent surface water control is best adapted to a reference location from the lower parts. Maximum values of estimated average gross return that can be obtained for both modes of surface water control are approximately the same. However, contrary to the traditional mode of surface water control, the maximum value for groundwater dependent surface water control is obtained almost without adversely affecting the farmers in the lower parts of the control area.
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