The urban water cycle

Abstract

Introduction The field of urban water management focusses on managing the (temporary) storages and flows of drinking, waste-, ground- , surface and storm water in urban environments. Adequate water management forms a prerequisite for pleasant living conditions . A difficulty urban water managers face is the limited or complete lack of control over some of the processes in urban areas, such as precipitation, evaporation, etc. Furthermore, the characteristics of an area in which the water needs to be managed forms a semi-fixed condition for the fluxes, as they cannot be manipulated to suit short term water management needs. Structural measures that can be taken in an area to improve the are relatively expensive and are therefore only possible once every (few) decade(s).This means that proposed changes to an area should result in similar or -preferably- improved living conditions for a long period of time. Next to the limited control, two other potential challenges to satisfactory management of water in urban areas are droughts and climate change.In order to be able to predict the merit of proposed measures during current and changed climatic conditions and droughts, more information is required on the possible consequences for the water cycle. Goal The goal of this research is therefore to investigate the relations between processes in the water cycle and their interdependencies. This thesis is also aimed at finding an indication of the possible effects of droughts and climate change on the urban water cycle. Approach For this study the water cycle of the Prinseneiland was used as a case study. The characteristics of the area were determined during a field survey. Further measurements on groundwater levels and sewer discharges were performed. Data on precipitation and potential evaporation were obtained from nearby measuring locations. A lumped, conceptual model was made for simulating the water cycle of the area. The model results were validated on the measured groundwater levels and sewer discharges. The model was used to simulate the effects of drought and climate change scenarios on the water cycle of the Prinseneiland. Conclusions Due to less precipitation in a dry year the interception evaporation, infiltration and surface runoff fluxes also decrease. Transpiration increases slightly due to higher potential evaporation. Groundwater recharge is decreases due to less infiltration and more transpiration. This causes the groundwater levels to drop more relative to normal situations. These effects are more or less proportional to the severity of the drought. Transpiration, however, is restricted by available soil moisture, which causes transpiration to remain relatively constant compared to the other fluxes. The effects of climate change mainly depend on whether the prevailing wind patterns change or not. In the latter case precipitation and potential evaporation increase, resulting in larger fluxes compared to the normal situation. When the wind patterns do change (as in the KNMI’06 ‘W+’ scenario) precipitation is concentrated in winter. During the summer months precipitation decreases and potential evaporation increases compared to the current situation. The interception evaporation, infiltration and surface runoff fluxes decrease. Transpiration increases due to higher potential evaporation. Groundwater levels are higher during winter due to higher winter precipitation and thus infiltration. In the summer groundwater levels decrease further compared to the normal situation.