Water redistribution at the soil surface : ponding and surface runoff in flat areas
toon extra info.
Willemijn M. Appels
|[S.l. : s.n.]|
|154 p fig., graf., tab|
|Proefschrift Wageningen toon alle annotatie(s)
Met lit. opg. - Met samenvatting in het Engels en Nederlands
|Zee, Prof. dr. ir. S.E.A.T.M. van der ; Bogaart, Dr. P.W.|
|Samenvatting door auteur||
Surface runoff is an important process that affects the local water balance and causes soil erosion and rapid solute transport towards ditches, streams, and rivers. Surface runoff is relatively rare in flat, agricultural areas in temperate climate zones and measuring and modelling the process is quite complicated. For this reason, there has been little systematic investigation into the controls on and the relations between driving mechanisms of surface runoff in The Netherlands, though surface runoff budget studies have been performed before. More specifically, the interactions between surface runoff controls and the role of the soil surface in transporting water to streams have not been given appropriate attention. The general aim of this thesis is to investigate and quantify the development of surface runoff on relatively flat soils of catchments in temperate climate zones.
In Chapter 2, a ponding and redistribution model is presented that simulates the flow of water over a surface with a heterogeneous microtopography with or without infiltration of water into the soil. With this model, the functional hydrological connectivity of synthetical elevation fields with varying statistical properties was investigated. The connectivity behaviour was determined by the presence of depressions with a large area compared to the field area and by the spatial organization of the microtopography in rills or channels. The presence of microdepressions suppressed the effect of the spatial variation of infiltration properties.
In Chapter 3, surface runoff measurements from two agricultural fields in the sandy part of The Netherlands are presented. These data were combined with auxiliary information to give an integrated narrative of surface runoff in flat, lowland catchments. During the seven events that occurred during the measurement period, up to 10 % of the event precipitation left the field as surface runoff. When the events resulted from a combination of rainfall and melting snow, the area contributing to surface runoff was small and located close to the field boundaries. When the events were driven by saturation excess, the area contributing to surface runoff was larger and flow paths were relatively long, because the wettest locations within the fields were found at a large distance from the ditches. In the days preceeding the events, ponding of water occurred in micro- and mesotopography at these wet locations. In this type of catchment, the formation of surface runoff was a two-stage process with thresholds from storage capacity of the unsaturated zone and the surface topography.
The ponding and redistribution model was extended with a 2D groundwater model to further examine surface runoff generation in fields with micro- or mesotopography under saturation excess conditions (Chapter 4). When the groundwater table is shallow, a small addition of water to the groundwater reservoir can lead to a large rise of the water table. These dynamics were included in the model. Under saturation excess conditions, the amplitude of the microtopography was more important for the surface runoff behaviour than its spatial structure. Mesotopography affected surface runoff development under saturation excess conditions by actually rerouting ponding water over longer distances. The infiltration of water in mesotopographic depressions decreased the gradient of the groundwater table over a large part of each field, thereby decreasing groundwater flow. The presence of mesodepressions effectively resulted in a nested hydrological system where smaller water bodies were activated when the amount of water stored in the system was large. Because of the nested activiation, the effect of the specific structure of rainfall series was large and lead to differences in total volume of surface runoff generated per season of one order of magnitude for statistically identical rainfall timeseries.
The model developed in Chapter 4 was applied to real fields with varying micro- and mesotopography in Chapter 5 with the goal of explaining differences in measured surface runoff volumes and nutrient loads of surface runoff between those fields. The groundwater dynamics and the depth of the unsaturated zone were found to account for a spatially variable initiation of ponding and surface runoff. However, this spatial variability could not be linked to the full meso- and microtopographies of the fields. Even though the model complexity was reduced, the non-linearities introduced by the dynamic specific yield, could only be treated at very small timesteps resulting in too long computation times.
In Chapter 6, the intricate relationships between microtopography, surface runoff, and ecohydrology in ecosystems with shallow groundwater tables are reviewed. The use of microtopography in modeling approaches was illustrated with two examples that feature a typical ecosystems with shallow groundwater under influence of microtopography. Microtopography could add flexibility to the acrotelm-catotelm concept in raised bog hydrology, by accounting for fill and spill processes along the sides of the bogs. Also, the effects of microtopography and surface runoff on the mixing of water with different chemical signatures and the subsequent results in variations of the occurrence of plant species were demonstrated.
In order to come to a good quantification of surface runoff in flat, well-drained agricultural catchments in temperate climate zones, the controls on its generating mechanisms and the feedbacks between those need to be understood. When the generating mechanisms in a field are properly quantified, the activation of flow routes through micro- and mesotopography can be determined.
The results presented in this thesis provide new insights into the complexity of processes and feedbacks involved in surface runoff generation on flat surfaces under conditions of infiltration and saturation excess. The results can be used to parameterize larger scale distributed models and suggest the steps that are needed to improve larger scale estimations of surface runoff risk and impact.
|Trefwoorden (cab)||afvloeiingswater / oppervlakkige afvoer / bodemwater / plasvorming / hydrologie / topografie|
|Hydrologie / Bodemfysica|
|Toelichting (Engels)||In The Netherlands, one of the most important targets for the improvement of surface water quality as aimed for in the European Water Framework Directive, is the reduction of nutrient concentrations (both nitrogen and phosphorus). To identify the most suitable and effective measures for reducing the tranport of nutrients from field to stream, it is important that the processes that control the transport are well identified and quantified. The PhD research presented in this thesis was part of the Alterra project “Relationships between groundwater and surface water”|
WUR, Sectie Waterhuishouding