Validation of the National Hydrologic Model (LHM) for the Fresh Water Demand in the Western Netherlands

Abstract

During dry conditions in the western Netherlands external fresh water needs to be supplied for surface water level maintenance, prevention of subsidence and peat oxidation and agriculture. The last decades already some serious dry years occurred (2003 and 2011) during which the fresh water demand was under pressure. Within the Deltaprogramma Zoetwater measures will be taken to secure a sustainable and sufficient fresh water supply. To determine the fresh water demand for the western Netherlands the LHM (Landelijk Hydrologisch Model) will be used. Based on the outcome of this model possible policy measures will be taken. This thesis validates the LHM to determine to what extent the LHM is usable and reliable to determine the fresh water demand for the western Netherlands during dry conditions. The base of this study is the setup of water balances, in which the inlet and outlet of water are the most important terms. Other parameters as precipitation deficit and seepage/infiltration are used to clarify possible deviations of the model. Differences in the amount of flush water between measurements and the LHM is corrected for by comparing the net drainage. It appears that the LHM calculates an almost correct water demand for the boezem of Rijnland and Delfland, with a sinusoidal deviation in water use throughout the year and an overuse of approximately 20-40mm during the growing season. The cause for the sinusoidal deviation could be a storage term in the model. In combination with conclusions from the ground water level validation, a possible location for this storage is the unsaturated zone. Most of the overuse in water during the growing season is depicted on the outlet of water; on average the deviation in inlet is smaller than the deviation in outlet. Additionally, not all of the over demand is concentrated on dry periods; during dry periods no extreme deviations in water demand are observed. However, imposed amounts of flush water in the LHM are not always correct, leading to large deviations in the inlet, especially for individual polders. Allocated agricultural surface water seems to be calculated on the low side, although the total water use for Rijnland and Delfland does apparently not show the same order of magnitude in deviation. For other regions in the western Netherlands measurement data was not detailed enough to make hard conclusions, but results showed the same trends as for Rijnland and Delfland. Except for the Amstel, Gooi and Vecht area and other regions with high seepage/infiltration rates. It appears that the LHM does not capture these extreme seepage/infiltration rates. The conclusion is that the HLM is usable for to determine the water demand for the western Netherlands, with exception of regions with high seepage/infiltration (>0.75mm/d). Additionally, one must be careful when selection small regions or short timescales, and the LHM results must be manually corrected to correct amounts of flush water.