Large-scale Hydrological Modelling in the Semi-arid North-east of Brazil

Semi-arid areas are characterized by small water resources. An increasing water demand due to population growth and economic development as well as a possible decreasing water availability in the course of climate change may aggravate water scarcity in future in these areas. The quantitative assessment of the water resources is a prerequisite for the development of sustainable measures of water management. For this task, hydrological models within a dynamic integrated framework are indispensable tools. The main objective of this study is to develop a hydrological model for the quantification of water availability over a large geographic domain of semi-arid environments. The study area is the Federal State of Ceará in the semi-arid north-east of Brazil. Surface water from reservoirs provides the largest part of water supply. The area has recurrently been affected by droughts which caused serious economic losses and social impacts like migration from the rural regions. The hydrological model WASA (Model of ater vailability in emirid Environments) developed in this study is a deterministic, spatially distributed model. Water availability is determined in terms of river discharge, storage volumes in reservoirs and soil moisture. Specific model formulations applicable for hydrological processes under semi-arid conditions are used. Temporal and spatial scaling approaches are applied to link the process scales with the scales of interest for model application. Modelling units are defined by similarity of landscape characteristics with regard to both lateral and vertical hydrological processes. Model applications of WASA generally result in a good model performance when compared to observed historical time series of river discharge and reservoir storage volumes. An appropriate process representation is shown to be the basis for reliable simulation results. However, the results are characterized by large uncertainties, mainly due to uncertainties of the input data in view of the low data availability. This refers in particular to rainfall data and soil parameters. Results of model simulations for climate scenarios until the year 2050 show that a possible future change in precipitation volumes causes a larger percentage change in runoff volumes by a factor of two to three. In the case of a decreasing precipitation trend, the efficiency of new reservoirs for securing water availability tends to decrease in the study area because of the interaction of the large number of reservoirs in retaining the overall decreasing runoff volumes.