Simulating the effects of reforestation on floods using spatially distributed hydrologic modeling and GIS

Scriptie/Masterproef: Doctoral Thesis


Flood as one of the worst natural disasters can among other facts be affected by land use changes. Land use and land cover are changing year by year due to the ever growing population and economy. The various types of land cover and land use have significant roles and impacts on runoff and flood; but how and to what extent is not clear and highly uncertain. Considering the advancement of simulation of hydrologic processes by distributed modeling applying GIS tools and using some remote sensing information, assessing these effects has increasingly become feasible in recent years. Computer simulation modeling has been used for at least 35 years to study the effects of land-use changes within catchments. The models are becoming more capable for flood prediction, land-use impacts on floods, and decision making in watershed management. The main objective of this PhD thesis is to investigate and to assess the impacts of land use changes (particularly reforestation) on floods by means of distributed modeling and GIS. A grid-based spatially distributed hydrologic model, WetSpa, which is developed at the Department of Hydrology and Hydraulic Engineering of VUB, is used in this research to fulfill the aim. The hydrologic processes considered in the model are precipitation, interception, depression, surface runoff, infiltration, evapotranspiration, percolation, interflow, groundwater flow, and water balance in the root zone and the saturated zone. The basic model inputs are a digital elevation model (OEM), land use and soil maps of the study area in GIS raster format, and hydrometeorological data. The model combines these data within GIS, and predicts flood hydrograph and the spatial distribution of hydrologic characteristics in a watershed. The WetSpa model uses a modified rational method to calculate runoff and degree-day coefficient method to estimate the snow melt runoff based on temperature data. The runoff is routed through the basin along flow paths using a diffusive wave transfer model that enables to calculate response functions between any start and end point, depending upon slope, flow velocity and dissipation characteristic along the flow lines. The model is applied to the Hamad river basin (4262 km\ and its main sub-catchments. The Homad River located in Slovakia, is one of the main tributaries of the Tisza River. Daily hydrometeorological data from 1991 to 2000, including precipitation data from 44 stations, temperature data from 14 stations and evaporation data measured at 4 stations were used as input data to the model. Three base maps, i.e. DEM, land-use and soil types were prepared in GIS form using 100 x 100 m cell size. Results of the simulations show a good agreement between calculated and measured hydrographs at the outlet of the basin and also in its main sub-basins. The model predicts the daily hydrographs with a good accuracy, between 75 to 80% according to the Nash-Sutcliffe criteria. For assessing the impact of land-use changes on floods, the calibrated model is applied for three reforestation scenarios using the hourly data of the summer of 2001. In the first reforestation scenario, the forest area is increased by 23%. The second scenario assumes that forest will increase by 38%, and for a third reforestation scenario it is assumed that forest will increase by 51 %. The model results show that the 51 % reforestation scenario decreases the peak discharge by 12%. Also, the time to peak of the simulated hydrograph of the reforestation scenario is 9 hours longer than for the present land-use. The results show that the effect of land cover on flood is strongly related to storm characteristics and antecedent soil moisture. A test using 3 rain scenarios demonstrates how the effect of the reforestation scenario differs from one storm to another depending upon the storm amount and duration. Moreover, sensitivity and uncertainty analysis of the model parameters is performed using a model-independent parameter estimator, PEST. It is found that the correction factor for calculating the actual evapotranspiration from potential evaporation has the highest relative sensitivity.
Datum prijs1 sep 2006
Originele taalEnglish
BegeleiderFlorimond De Smedt (Promotor), Marc Van Molle (Promotor), Jan Wastiels (Jury), Jean Vereecken (Jury), Willy Bauwens (Jury), Geza Jolonkai (Jury), Okke Batelaan (Jury) & Peter Roncak (Jury)

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