Model for Water Availability in Semi-Arid Environments (WASA) Estimation of transmission losses by infiltration at rivers in the semi-arid Federal State of Ceará (Brazil)

The hydrological model WASA (Model of Water Availability in Semi-Arid Environments) has been developed as a deterministic, spatially distributed model, consisting of conceptual, process-based approaches, to quantify water availability in large semi-arid regions. Water availability can be calculated with a daily resolution and on different spatial levels. The model has been applied by Güntner (2002) to the study area "Federal State of Ceará", a 150.000 square-kilometre large region in the semi-arid northeastern part of Brazil. The mean annual precipitation of 850 mm occurs generally during a rainy season of five months (Güntner: XVII). As precipitation in semi-arid tropical areas is mainly of convective type and rainfall events characterized by short duration, high intensities, large spatial and temporal heterogeneity, river channel runoff is correspondingly variable. In the study area, river flow occurs only periodically during the rainy season. Besides this seasonal variation of river flow, the intermittency is also due to the lack of important groundwater reservoirs which could provide base flow and avoid influent conditions of rivers (Güntner: 6). In WASA spatial units are connected by a dendritic river network and the routing process of river runoff through each of these units is approximated by a daily response function. Up to now, the routing process in WASA considers only transmission losses by evaporation from the surface of the river network. Transmission losses by infiltration are not taken into consideration in routing processes yet, although the volume of these supplementary losses under the aforementioned conditions seems to be important and affects several hydrological processes as runoff volume and peak discharge, groundwater recharge etc.. In this study an attempt to consider transmission losses by infiltration and implement a corresponding approach in the river routing process of WASA has been undertaken and will be described in the following chapters in detail. 2 Transmission losses "When flow occurs in normally dry stream channels, the volume of flow is reduced at downstream points by evaporation and infiltration to the bed, the banks, and possibly the flood plain. This flow reduction is termed transmission losses. Quantification of transmission losses yields useful information not only on surface runoff volumes, but also on ground water recharge in some regions." (Walters: 129) The two dominant processes forming transmission losses are infiltration and evaporation. Transmission losses by evaporation are actually considered in the WASA model, subtracting in each sub-basin transmission losses from the river runoff volume. These evaporation losses are …