Water withdrawn from Tescou and Seoune rivers for the irrigation, its consequences on the hydrological regime of the sub-basins (Adour-Garonne watershed, France)
The water needs for farming irrigation in the Tescou and Séoune sub-basins (Basin Adour-Garonne) have increased considerably over the past thirty years. The needs were met, in part, by the creation of numerous reservoirs. For the Tescou catchment, with rapid but small-volume floods, we have 184 individual reservoirs with an accumulated theoretical volume of 4.3 Mm3, which intercept about a third (92 km2) of its surface (287 km2). For the Séoune catchment, the need for irrigation is a little less strong, considering its size (463 km2) and the presence of a relatively more abundant water resource. Close to 160 reservoirs can be counted, with a theoretical storage volume of 6.5 Mm3 for an intercepted surface of about a quarter of the sub-basin (122 km2). Hydrological monitoring data are available in the form of an influenced time series Q(t) with a period of observation of about 30 years for each sub-basin. From the observed discharges, our objective was to identify and to quantify anthropogenic impacts on three components of the hydrological regime: floods, annual mean discharge and low-flows. The seasonal approach of floods and annual mean discharges permitted a differentiation of the anthropogenic impact according to the "winter" (months 12 to 6) and "summer" (months 7 to 11) seasons. This approach reflects the present management of water in these agricultural sub-basins: storage in winter and irrigation during the first months of summer. To identify the impact, we used tests to detect stationnarity breaks in the time series: LANG (2004) for floods and an adaptation of BOIS (1986) for annual mean discharges and low-flows. For summer floods and summer annual mean discharges, the tests showed that the time series were stationary during the period of observation. For winter floods and winter annual mean discharges the tests showed stationnarity breaks in the time series, which allowed us to define the stationary sub-periods. These sub-periods were consistent with the evolution of the accumulated theoretical withdrawals during the last thirty years. The same analysis of stationnarity breaks made on flows was also done on rainfall to strengthen our results. The stationnarity of the rainfall observed over the time series shows that there is not a climatic component; in other words, the trends detected in the streamflow record are essentially of anthropogenic origin. For both sub-basins, the BOIS statistical test did not reveal any impact of possible pumping on the low-flows, and especially for the VCN30 norm (minimum annual mean flow over a period of 30 consecutive days). Finally, the use of water for irrigation essentially affects winter floods and annual mean discharges. A test to quantify this trend was carried out with a statistical analysis associating the observations of each stationary sub-period and the discharges simulated for a similar rainfall. From the resulting statistical models and the curves of pressure of theoretical irrigation requirements, we deduced a probable trend for the quantiles of winter floods and mean annual winter discharges. In a general way, winter floods in both sub-basins were strongly reduced in peak and volume by the set of the small reservoirs. The impact observed on the flood quantiles remains coherent in relation to the theoretical withdrawal volumes referenced above. The reservoirs are without significant action on the transfer time (approximately 19 days) in the Séoune sub-basin, where the floods are voluminous. On the other hand, for the Tescou sub-basin, with rapid and small-volume floods, the time of transfer increases considerably (8 h) between the state of zero withdrawal (natural: 1.73 days) and the present state close to 6 Mm3 (2.07 days). Concerning winter annual mean discharge in both sub-basins, the winter mean runoff coefficient decreased 31% and 42%, for the Séoune and Tescou sub-basins respectively, between the natural state and the present state. This reduction of the runoff coefficients does not correspond to the theoretical volumes stocked in the reservoirs. In other words, the theoretical volumes of the reservoirs cannot completely explain the observed strong reduction of the runoff coefficients. However, the consistency of the various controls, both on the observed time series and the modelled results, lend credibility to the overall results. These results should be taken into consideration in water management, if the climatic conditions and soil occupation don't change.