Modelling hydrological impacts of climate and irrigation changes in a mediterranean catchment

Climate change will have strong impacts on agriculture and hydrological processes, especially in semi-arid regions, where water resources are limited. However, these impacts are difficult to quantify. Apart from the uncertainties directly related to climate predictions (notably the prediction of future precipitation), the evaluation of hydrological effects of climate change also depends on the accurate simulation of water fluxes on the catchment scale, which are often uncertain. To simulate the catchment responses to climate change, different types of hydrological models have been used in the past, with various degrees of success. In this thesis, my aim is to evaluate the usefulness of integrated hydrological models to predict the impacts of climate change on water resources in semiarid, Mediterranean catchments. Integrated hydrological models, or pde-based models, are distributed hydrological models based on partial differential equations (pde). They usually simulate subsurface flow based on the Richards’ equation and surface flow based on a simplification of the shallow-water equations. They account for surface-subsurface interactions in a spatially-distributed manner and they represent the main hydrological processes (infiltration, exchanges in the hyporheic zone, evapotranspiration, etc.) with some level of detail. However, they generally have high computational needs, which result in long simulation times. These long simulation times limit the applications of integrated hydrological models in realistic case studies, particularly because the model calibration and uncertainties qualification require a large number of model runs. Because of the long simulation times and the large number of model parameters in many integrated hydrological models, model calibration can be a lengthy and tedious process. Hence, an important criterion to evaluate the practicability of integrated hydrological models in climate-impact studies is the feasibility of the model calibration. To accelerate the calibration procedure, I propose a method which is based on a set of computational grids of increasing resolution. I test this method in a particular case study, the Lerma catchment, situated in the Ebro basin (north-east Spain). In this case study, the model calibration is about eight times faster using the nested method than a direct manual calibration. Nevertheless, the model calibration is still considerably slower (about one month on a desktop computer) than the calibration of simpler conceptual models. An important characteristic of the catchment which is modeled in this study is the rapid transition from rainfed to irrigated agriculture between 2006 and 2008. The calibrated hydrological model generally reproduces this transition with respect to changes in discharge and hydraulic heads. Consequently, this model is a valuable tool when studying the interactions between climate and irrigation changes, which are important to understand future impacts of climate change. For this purpose, I model the hydrological responses of the Lerma catchment to simultaneous climate and irrigation changes. To this end, climate scenarios for the region are developed, based on four regional climate models from the European ENSEMBLES project (http://www.ensembles-eu.org). The outputs from the regional climate models are downscaled using a calibrated weather generator and a quantile-based method. Based on the results from the hydrological simulations, it is found that responses to climate change significantly depend on the irrigation scenarios. Hydraulic heads and base flow are larger under irrigated conditions, but they are more sensitive to climate change when irrigation is present. In contrast, responses of annual maximum flow to large precipitation events are more intense in the scenario without irrigation. In future climate, actual evapotranspiration in summer decreases under nonirrigated conditions due to the lower summer precipitation, and increases under irrigated conditions because of the larger soil moisture, which results in a higher sensitivity of actual evapotranspiration to temperature. This analysis would have been difficult, or even impossible, to conduct with other types of hydrological models. In addition to mean climatic conditions, droughts are also examined. Droughts are a serious risk for the Ebro region notably because of the economical importance of the agricultural sector, which has large water needs. Changes in future droughts are often studied from a meteorological point of view. Drought indices (i.e., a single numerical value representing the dryness level based on one or more hydro-meteorological state variables) are computed based on the outputs from regional or global climate models in present and future climate. Differences in drought indices are then used to predict future drought conditions. I compare the results from this approach to the outputs of the integrated hydrological model during dry periods. Specifically, I examine the correlation between seven well-known drought indices and three modeled hydrological variables in the climate and irrigation scenarios used previously to study the interactions between irrigation and climate changes. In addition, I control the validity of the functional relationship between these indices and the hydrological variables under the predicted, future conditions. Correlations between the hydrological variables and drought indices are similar in all studied conditions. However, the relationships between the drought indices and the hydrological variables are often different in present and future climate. Hence, drought indices are adequate indicators of dry periods in all climates. However, a drought with a similar intensity (as defined by a particular drought index) might have different hydrological impacts in present and future climate. These differences cannot be captured by studying meteorological drought indices alone. To conclude, integrated hydrological models have important advantages over other types of hydrological models in climate-impact studies: They allow for a joint analysis of surface and subsurface flow, they can be used to study interactions between land-use and climate changes, and they can be employed to test simpler methods, such as the use of drought indices. However, their use is hindered by their high computational needs. This obstacle can partially be overcome by simplifying the computational grid during part of the calibration process.