Runoff as an additional water resource in a semiarid karstic aquifer

In a semi-arid limestone area, finding additional water resources is a priority. Moreover in the Sierra de Gador-Campo de Dalias, the local economy is booming and mainly based on groundwater resources. In common with other semi-arid to arid regions in particular along both sides of the Mediterranean are facing the same difficulties in balancing economic and natural resources. Aquifers are often overexploited and the demand is still growing. Hence, for sustainable development, water resources must be evaluated. Long term estimates are generally based on global assessment of diffuse recharge. This is somehow paradoxal as extreme events are recognized to preferentially recharge the aquifer. We show that the selectivity of deuterium excess to monthly precipitation volume allowed estimating the return period of precipitation leading to aquifer recharge at 0.9-4.9 years. Still moderate rainfall, which occurs more frequently, represents c. 60-90% of the total precipitation volume. This thesis aims to contribute to the understanding of the groundwater recharge system with particular attention to runoff processes and a focus on runoff as a component of water balance in semi arid areas. Runoff is usually regarded as negligible in the recharge system of the semi-arid aquifers. Generally rare and discontinuous, the runoff process is difficult to monitor. The approach used is to first evaluate the main factors explaining the occurrence of runoff reaching the bottom of the hillslope for several representative sites in the study area (0.3-20 ha). It was demonstrated that runoff events depend on thresholds i.e. rainfall depth per event, the antecedent precipitation index over 20 days (API20) and soil surface characteristics. Then, a simple rainfall/runoff model was adapted and validated at the hillslope scale. The expert-based model allowed acceptable results for predicting runoff at this scale. Finally, the hillslope model was extrapolated to the catchment scale (26 km²) in order to estimate the impact of runoff as an additional source of surface and groundwater. The comparison between recharge determined by means of tracer techniques reported in the literature for the same catchment and concentrated recharge from our model results revealed that runoff that infiltrates in ephemeral stream contributes to a non negligible volume of about 10% of the total recharge.(GEOG 3) -- UCL, 200

Predicting runoff from semi-arid hillslopes as source areas for water harvesting in the Sierra de Gador, southeast Spain

The effectiveness of water harvesting systems collecting surface runoff form rangeland hillslopes in semi-arid regions is difficult to predict, since the hydrological response at the outlet depends on the heterogeneity of hydrological processes. The lack of continuous runoff pathways, due to the irregular spatial patterns of soil properties and the variety of antecedent soil moisture conditions directly influence runoff generation and control discharge into the water harvesting cisterns. The aim of this paper is to evaluate the effectiveness of semi-arid hillslopes in generating runoff for water harvesting systems. Runoff was estimated by the STREAM expert-based model which was applied to three semi-arid hillslopes (0.4 to 6 ha). On the one hand the STREAM model rules were adapted to the regional conditions i.e. an antecedent precipitation index was adjusted to local soil moisture conditions and the rainfall duration was defined as the total rainfall event quantity and the effective rainfall duration (P-tot/t(eff)). On the other hand, the distribution of rock outcrop and vegetation cover along the slopes was used to define homogeneous hydrological units. Final infiltration capacities were attributed to these hydrological units based on values found in the literature. The prediction performances are acceptable for the three water harvesting systems with an RMSE of 13.9 m(3). It was shown that the rainfall/runoff model was more sensitive to the duration of the storm than to the antecedent soil moisture conditions. The use of a unique set of hydrological parameters for the three water harvesting systems on representative hillslopes allows the runoff prediction from any rangeland hillslope within the same region. Furthermore, the spatial patterns of soil surface characteristics are crucial for collecting runoff at the outlet of the system. Model runs demonstrated that degradation of vegetation and sealing of very small areas within flow paths can lead to an increase of annual runoff by as much as a factor two. (C) 2009 Elsevier B.V. All rights reserved

Water harvesting potential in function of hillslope characteristics: A case study from the Sierra de Gador (Almeria province, south-east Spain)

Surface water in limestone and dolomite mountain ranges in semi-arid regions is very rare. Therefore, water harvesting systems, collecting the runoff from hillslopes, can be found at regular distances to supply water points for grazing animals. The hydrological behaviour of a range of hillslopes and small catchments call thus be studied by monitoring the rainfall and runoff for such water harvesting systems. The number of runoff events collected in four of such systems ranged from 2 to 22 per year during a 7-year period in the Sierra de Gador (Almeria province, Spain). The large variation in runoff response can be attributed to the distribution of the rainfall (long-term average 400 mm) and the characteristics of the catchment areas. Rainfall-runoff behaviour was far more erratic than reported for plot studies and rainfall simulations in other semi-arid regions, demonstrating that plot scale results cannot be extrapolated to design water harvesting systems. The aim of this study was to determine thresholds for the production of runoff oil hillslopes and zero-order catchments reaching the water harvesting reservoirs at the footslopes. Multiple logistic regressions showed that the rainfall depth per event and the antecedent precipitation index over 20 days (AP120) are the main parameters to explain the occurrence of runoff in the reservoirs connected to the four catchments. The hillslopes show a systematic pattern of rock outcrops on the crest and shoulders to a colluvial mantle at the footslopes, where the cisterns are dug out. The rainfall and API threshold of the four catchments, required to produce runoff in the reservoirs, are lowest for hillslopes with a short distance from the rock outcrops to the reservoirs and a low vegetation cover. It was demonstrated that (i) re-infiltration of runoff frequently occurs (56 events not reaching the reservoirs against 14 events with continuous runoff) even within the unlined channel connecting the cistern to the hillslope, and that (ii) tile 80% probability of runoff occurrence, calculated using the logistic regression, can be used as a proxy for the total annual runoff to be collected. (c) 2008 Elsevier Ltd. All rights reserved

Origin and type of rainfall for recharge of a karstic aquifer in the western Mediterranean: a case study from the Sierra de Gador-Campo de Dalias (southeast Spain)

Isotope signatures in precipitation from the Global Network for Isotopes in Precipitation around the Mediterranean basin and literature data are compared with isotopic data from a large karstic aquifer in southeast Spain to explain the origin and type of the precipitation events dominating recharge. Analysis of the deuterium excess d at the scale of the Mediterranean basin and at the regional scale allows us to understand the isotopic context of the study area: Campo de Dalias and the Sierra de Gador (Almerfa province). The origin of precipitation can be determined from its d value. The d value changes as a function of the initial evaporation condition. It depends on the relative humidity and temperature during the evaporation producing the water vapour of the clouds. The water vapour, which dominates the study area, is generated in two areas: the Atlantic Ocean (d = 10 parts per thousand) and the western Mediterranean basin (d = 15 parts per thousand). With increasing precipitation volume, the western Mediterranean character dominates. These heavier storms contribute mainly to recharge, as illustrated by the d value of 13.6 parts per thousand in deep groundwater of the Campo de Dalias. Weighted d values increase with the volume of precipitation, giving a significant relationship for the southern and eastern coasts of the Iberian Peninsula. This selectivity of d to monthly precipitation was used to estimate the return period of precipitation leading to aquifer recharge at 0.9-4.9 years. Moderate rainfall, which occurs more frequently, still represents similar to 60-90% of the total precipitation. One of the challenges to meet ever-growing water demands is to increase recharge from moderate events yielding intermediate quantities per event, but forming the bulk of the annual precipitation. Copyright (c) 2006 John Wiley & Sons, Ltd

Using stable isotope analysis (delta D-delta O-18) to characterise the regional hydrology of the Sierra de Gador, south east Spain

Water stress is rapidly increasing in many Mediterranean coastal zones mainly due to expansion in agriculture and tourism. In this paper, we focus on the Sierra de Gador-Campo de Dalias aquifer system (southeastern Spain) in order to assess the capability of water stable isotope analysis (deltaD-delta(18)O) to refine the understanding on recharge of this karstic aquifer system. Different types of surface and groundwater were sampled along an altitudinal gradient from the recharge zone in the mountains to the coastal plain. Surface water is restricted to local runoff, collected in closed reservoirs. Runoff amounts, collected in three of these reservoirs were monitored together with the precipitation in their catchments. Meteorological maps were used to detect the origin of the precipitation generating the majority of the runoff. The results were compared to literature data on local and regional precipitation. The use of oxygen and hydrogen isotopic composition has proved to be a useful tool to explain the origin of groundwater in a Mediterranean karstic system. Such studies are, however, not numerous and are often limited to local scale recharge for fast-reacting systems. This paper focuses on the delta(18)O-deltaD relationships of local precipitation to explain the isotopic variability of a large karstic aquifer system. The isotopic compositions of groundwater sampled along an altitudinal gradient from the recharge zone to the coastal plain are well displayed, in a deltaD-delta(18)O diagram, on a mixing line connecting a pole of Mediterranean waters to a pole of Atlantic waters. The Atlantic signature predominates in the shallow groundwater of natural springs, reflecting the rainfall which produced the local runoff sampled. The Mediterranean signature is mainly restricted to deep groundwater from boreholes in the coastal plain. The existence of a degree of spatial separation of groundwater types demonstrates that groundwater flow in a complex karstic system is not always continuous. The Mediterranean signature of deep groundwater could be due to past extreme rainfall events during which connectivity between recharge and reservoir exists, while at the same time the Atlantic signature of recent winter rains dominates in shallow groundwater. The assumption that an equilibrium in isotopic composition is established within a continuous aquifer and that therefore a slope lower than 8 in a deltaD-delta(18)O diagram indicates evaporation is not necessarily valid. (C) 2002 Elsevier Science B.V. All rights reserved