Abstract
The RUT Irrigation District is a flat polder area with an infrastructure for irrigation, drainage and flood protection. Water is pumped out for drainage, mainly during the wet season and is pumped in for irrigation during the two growing seasons. The RUT Irrigation District is one of the 16 districts that were transferred by the Colombian government to a water users' association. During this process the farmers agreed to take over the management and to give up governmental subsidies. It appeared that the farmers grossly underestimated the future energy costs and nowadays they complain about high costs for operation and maintenance of the pumping stations. In addition, the service provided is not adequate and the condition of the pumps is declining.
The supplementary irrigation is based on the rainfall deficit, but a distinct criterion to supply water according to daily rainfall and cropping calendar does not exist. Hence, it is possible that more water than required is supplied to the area, affecting not only the pumping costs for irrigation but also for drainage. The present situation has been evaluated with a water balance at scheme level and an analysis of the measured groundwater fluctuations. Consecutively, a water balance at field level with an unsaturated groundwater flow model has been simulated to determine the irrigation requirements, yield reduction and drain flow based on a 20βyear analysis. The water balance at field level and the oneβ, twoβ and threeβday rainfall with a return period of 5 years resulted in design drain flows. Finally, these flows were used in hydrodynamic simulations to analyse the pumping requirements for optimal drainage. The simulations also indicated the inundated areas to be expected for different rainfall intensities and frequencies. Copyright Β© 2004 John Wiley & Sons, Ltd.