Cristina Aguilar

Assessing minimum environmental flows in nonpermanent rivers

The criteria used in the computation of the minimum environmental flow regime and flow cessation periods in nonpermanent rivers are often left to open criteria. This study proposes a stochastic approach for evaluating the choice of local thresholds in the characterization of minimum environmental flows through both the Monte Carlo technique and local hydrological relationships. This approach is applied to four regimes obtained by hydrologic and hydraulic habitat modeling in a Mediterranean watershed. The operationality, defined as the probability of the calculated environmental regime being satisfied by the natural regime over 25 years, was assessed for eight different scenarios. Two monthly minimum environmental flow regimes were then generated, with 90 and 95% operationality levels. This analysis allows the generation of minimum flow regime prescriptions from a strictly hydrologic point of view. The methodology proposed constitutes a useful tool for the implementation of uncertainty analysis of environmental flows in water resource management. Minimum environmental flows are sensitive to thresholds applied in the calculations.We propose a stochastic approach for determining local thresholds.Optimization was applied to give prescriptions based on an operationality assessment.A tool for the uncertainty assessment required in risk analysis is provided.

NDVI sensitivity to the hydrological regime in semiarid mountainous environments

This work shows the sensitivity of NDVI as an indicator of the global hydrological regime of the year. The annual water balance in the area was simulated through a physically-based distributed hydrological model previously calibrated and validated in the area from 2001 till 2010. NDVI was obtained from Landsat TM at the end of the dry season in 1000 points randomly distributed over a pine cover in a mountainous Mediterranean area. The influence of different hydrological processes related to the water balance in the soil on the NDVI values was analyzed through Pearson correlation matrices and Principal Components Analyses. Results showed that the NDVI was particularly sensitive to the regime of annual variables related to the snow layer dynamics, especially to snowmelt. These relationships were quantified, with the best fit being obtained between NDVI and the dimensionless index snowmelt divided by precipitation (R2 around 0.7). The adjustments obtained could, in the future, constitute a tool for the estimation of hydrological variables from satellite data in data-poor situations conditioned by the commonly steep slopes and difficult access in mountainous areas.

Local effects on the water balance in flood plains induced by dam filling in Mediterranean environments

Dams are common structures in order to guarantee water supply and control flash floods in Mediterranean mountainous watersheds. Even though they are known to modify in space and time the natural regimen of natural flows, little has been said about local effects on the ecosystem along the river banks upstream the dam. In 2002, Rules dam (southern Spain) started to function. This work presents the effects of the dam filling on the water balance in flood plains. The influence of the enhanced soil moisture in the surroundings of the free surface of the reservoir on the vegetation cover status was analyzed and related to meteorological agents and topographic features, before and after the construction of the dam. Meteorological, topographic, soil and land use data were analyzed in the contributing area of the dam, together with Landsat TM images during the period 1984-2010 to derive NDVI values. Results showed higher NDVI values (close to 20-30%) once the dam was filled and NDVI values in very dry years similar to the ones obtained in medium-wet years prior to the construction. Besides, NDVI values after the filling of the dam proved to be highly related to meteorological variables. Principal Component Analysis (PCA) was carried out in order to identify individual and combined interactions of meteorological and dam-derived effects. 85% of the total variance can be explained with the combination of three Principal Components (PC) in which the first one includes the combination of NDVI, meteorological (rainfall) and hydrological variables (interception, infiltration, evapotranspiration from the soil), whilst the second and third PC mainly include topographic features. These results quantify the dam influence along the river banks and the superficial recharge effects in dry years.