Modelling the rainfall-runoff response from a headwater wetland

Abstract

In southern Ontario, treed headwater swamps are a common watershed feature. These headwater swamps commonly exist at groundwater discharge sites and represent a significant link between the underlying groundwater system and the surface drainage system. In contrast to the volumes of literature pertaining to the hydrologic modelling of agricultural and forest land classes, relatively little attention has been focused on the development and testing of numerical simulation models suitable for predicting the stormflow response from these headwater wetland sites. If required to predict the rate of outflow from a wetland-dominated catchment, the hydrologist or engineer has few numerical tools and little data available to assist in the prediction.

From a modelling perspective, wetlands ecosystems represent a unique and complex hydrologic setting. The rainfall-runoff response from wetlands is shaped by numerous factors including the size and shape of the wetland, the topography of the wetland, the available air-filled pore space within the sediments and the characteristics of the drainage network. The movement of water through the wetland occurs as a combination of subsurface flow through the organic sediments and surface flow within the hummock terrain common to most wetlands.

The objective of this research is to investigate the feasibility of applying a numerical model to simulate the rainfall-runoff response from a treed headwater wetland site. A first-generation wetland model is developed in order to provide an operational tool capable of reproducing the hydrologic behaviour of the headwater wetlands common to southern Ontario. The process representations incorporated in the model structure are consistent with the data-poor environment typical of wetland systems and the computational requirements appropriate to modelling at the catchment or watershed scale. The wetland model utilizes a field hydrology model coupled to the stream routing model. The field hydrology model incorporates process representations for the horizontal movement of water through the wetland sediments and surficial hummock terrain. The channel routing model provides an accounting of the lateral exchange of water with the wetland sediments and simulates the transport of water through the wetland drainage network.

The development and testing of the wetland model is made in conjunction with a data collection program where hydrometric and meteorologic data were obtained at a 400 hectare first-order headwater swamp located within the Teeswater River watershed in southern Ontario. Field surveys of the wetland drainage network and a stream gauging program provide data pertaining to the open channel flows within the wetland. The observed rainfall-runoff response of the wetland clearly reveals a remarkable variability in the stormflow response.

As plot sample approach is used to calibrate the model parameters and validate the model performance. The ability of the model to simulate the runoff response from the headwater swamp is investigated for discrete storm events and for monthly simulations. Properly calibrated and initialized, the wetland model provides a useful tool for simulating the stormflow response for short to medium length simulations. As a result of the long response time associated with the drainage of wetlands and the difficult of quantifying the role of the underlying groundwater system, long-term simulation of the wetland stormflow dynamics is a more challenging task.

The sensitivity of the model output to changes in the calibrated model parameters is reported for two storm events: one event dominated by flows within the organic sediments and another storm in which the response is dominated by surface flows within the hummock layer. The results of the analysis indicate that the stormflow response from the wetland is significantly influenced by the antecedent wetland saturation and the precipitation input.

Overall, the use of an idealized wetland representation involving a stream routing function coupled to a wetland field model appears to hold promise for the simulation of the stormflow response of low-gradient headwater wetlands. However, the testing of the proposed model has been limited to a single wetland site. A true measure of the usefulness of the developed wetland model can only be assessed through continued application of the model to other wetland sites as additional hydrologic data are made available.