During my internship by Deltares I researched how often surface runoff do occur and which processes are decisive if this will occur or not. To find out I did fieldwork, modelling and data analyses for a field site in Eastern part of Twente (Netherlands).
The subject of this study is surface runoff. The main research questions that are addressed in this study is how often does surface runoff occur and what are the most important processes which determine if surface runoff will occur? The study area is located in the east of Twente in the Netherlands. Along the field side a nature area is located and within this area a brook is present called ‘Springendalse beek”. This brook is incising in the subsoil, because of the effect of peak flows and therefore the brook will drain the direct surrounding more compared to the past. The result is a dryer surrounding which causes unwanted changes in the vegetation. The field site is located on a moraine with a highly variable subsoil. In this area Glacial Till is very important for the occurrence of surface runoff and therefore the subsoil was mapped in detail. During the mapping of the subsoil the depth of the Glacial till was found to be very variable. Based on this new knowledge, a groundwater model was developed with iMODFLOW (based on MODFLOW) to simulate by what kind of precipitation surface runoff occurs. In addition, a field measuring campaign was set up to measure surface runoff. Water samples of surface runoff and groundwater were collected during dry and wet conditions. During wet condition the ponding was mapped and during dry condition infiltration test were done. Also an analysis is made of a historical data set from the flow of a brook that discharges a big part of the field side. The result of this research show that more often surface runoff occur during winter compared to summer, but in the summer surface runoff occurs more than expected. In summer the cause of surface runoff is exceedance of the infiltration capacity of the soil while in winter high groundwater tables play an important role. On locations where glacial till is shallow in the subsoil apparent water table will occur by precipitation surplus, caused by the low infiltration rate of the glacial till.