L. Villagarcía

Evaluating the long-term water balance of arid zone stream bed vegetation using evapotranspiration modelling and hillslope runoff measurements

The difference between long-term actual evapotranspiration (AET) and precipitation (P) provides a useful indication of the extent to which a site retains or loses water resources, and therefore of the likely occurrence of specific land degradation processes. Sink areas OAET q PU receive lateral water inputs from other parts of the catchment. In arid and semi-arid environments these areas are frequently found in, or next to, the stream beds of ephemeral rivers and are often characterised by intensive land use or high conservation values. For both types of land use it is important to know if, and how much, AET exceeds P, and where the lateral water inputs come from. Thick sedimentary fills in the stream bed, variable climate conditions and ephemeral flow conditions pose specific difficulties to the evaluation of the water balance of these sites. The objective of this study was to develop an approach to explore the relative importance of lateral water inputs to shrub stands growing in thick sedimentary fills of semi-arid ephemeral rivers. The approach is based on (i) estimating long-term AET2 P balances in the channel sediments and (ii) assessing whether these inflows originate mainly from surrounding hillslopes or from the upstream part of the catchment. A physically based evapotranspiration model for sparse vegetation was used to estimate the long-term AET rates. The relative importance of hillslope runoff and channel flow was evaluated in a semi-quantitative fashion from a combination of surface area estimates and mostly published values of soil hydrological parameters. The approach was developed and tested in a selected stand of Retama sphaerocarpashrubs in a stream bed at the Rambla Honda field site (Tabernas,

Variations in daytime net carbon and water exchange in a montane shrubland ecosystem in southeast Spain

Carbon and water fluxes in a semiarid shrubland ecosystem located in the southeast of Spain (province of Almería) were measured continuously over one year using the eddy covariance technique. We examined the influence of environmental variables on daytime (photosynthetically active photons, FP >10 µmol m−2 s−1) ecosystem gas exchange and tested the ability of an empirical eco-physiological model based on FP to estimate carbon fluxes over the whole year. The daytime ecosystem fluxes showed strong seasonality. During two solstitial periods, summer with warm temperatures (>15 °C) and sufficient soil moisture (>10 % vol.) and winter with mild temperatures (>5 °C) and high soil moisture contents (>15 % vol.), the photosynthetic rate was higher than the daytime respiration rate and mean daytime CO2 fluxes were ca. −1.75 and −0.60 µmol m−2 s−1, respectively. Daytime evapotranspiration fluxes averaged ca. 2.20 and 0.24 mmol m−2 s−1, respectively. By contrast, in summer and early autumn with warm daytime temperatures (>10 °C) and dry soil (<10 % vol.), and also in mid-winter with near-freezing daytime temperatures the shrubland behaved as a net carbon source (mean daytime CO2 release of ca. 0.60 and 0.20 µmol m−2 s−1, respectively). Furthermore, the comparison of water and carbon fluxes over a week in June 2004 and June 2005 suggests that the timing—rather than amount—of spring rainfall may be crucial in determining growing season water and carbon exchange. Due to strongly limiting environmental variables other than FP, the model applied here failed to describe daytime carbon exchange only as a function of FP and could not be used over most of the year to fill gaps in the data.

Temporal dynamics of soil water balance components in a karst range in southeastern Spain: estimation of potential recharge

Abstract This paper analyses the temporal dynamics of soil water balance components in a representative recharge area of the Sierra de Gádor (Almeria, southeastern Spain) in two hydrological years. Two approaches are used to estimate daily potential recharge (PR): Approach 1 based on deriving PR from the water balance as the difference between measurements of rainfall (P) and actual evapotranspiration (E) obtained by eddy covariance; and Approach 2 with PR obtained from the dynamic pattern of the soil moisture (θ) recorded at two depths in the sites thin soil (average 0.35 m thickess). For the hydrological year 2003/04, which was slightly drier than the 30-year average, E accounted for 64% of rainfall and occurred mainly in late spring and early summer. The PR estimated by Approach 1 was 181 ± 18 mm year-1 (36% of rainfall), suggesting an effective groundwater recharge in the study area. In the unusually dry hydrological year 2004/05, E was about 215 mm year-1, close to the annual rainfall input, and allowing very little (8 ± 12 mm year-1) PR according to Approach 1. Estimation of PR based on Approach 2 resulted in PR rates lower than those found by Approach 1, because Approach 2 does not take into account the recharge that occurs through preferential flow pathways (cracks, joints and fissures) which were not monitored with the θ probes. Moreover, using Approach 2, the PR estimates differed widely depending on the time scale considered: with daily mean θ data, PR estimation was lower, especially in late spring, while θ data at 30 min resolution yielded a more reliable prediction of the fraction of total PR resulting from the downward movement of soil water by gravity. Citation Cantón, Y., Villagarcía, L., Moro, M. J., Serrano-Ortíz, P., Were, A., Alcalá, F. J., Kowalski, A. S., Solé-Benet, A., Lázaro, R. & Domingo, F. (2010) Temporal dynamics of soil water balance components in a karst range in southeastern Spain: estimation of potential recharge. Hydrol. Sci. J. 55(5), 737–753.

Estimating Latent and Sensible Heat Fluxes using the Temperature Vegetation Dryness Index and MODIS Data

In this work, daily latent heat (lambdaE_d) or evapotranspiration and sensible heat (H_d) fluxes were estimated from the TVDI (Temperature Vegetation Dryness Index) modified to account for climatic gradients (TVDI_t) using MODIS data in a Spanish region (Andalusia) with strong bioclimatic gradients. The TVDI_t was correlated (R=0.90) with field measured AWC (available water content). When reinterpreting TVDI_t as the ratio between actual and potential evapotranspiration to estimate surface energy fluxes, model vs. eddy covariance data from 2 semiarid sites were reasonable for H_d (R=0.94: RMSE=25.89 Wm^-2). For lambdaE_d RMSE was low (19.46 Wm^-2) but correlations were only significant after excluding summer dates (R=0.71) when transpiration is very low. Model accuracy is currently limited by the accuracy in Rn_d estimates (<27 Wm^-2) for which results at humid sites should be better than at semiarid sites.

Vertical and lateral soil moisture patterns on a Mediterranean karst hillslope

Abstract The need for a better understanding of factors controlling the variability of soil water content (θ) in space and time to adequately predict the movement of water in the soil and in the interphase soil-atmosphere is widely recognised. In this paper, we analyse how soil properties, surface cover and topography influence soil moisture (θ) over karstic lithology in a sub-humid Mediterranean mountain environment. For this analysis we have used 17 months of θ measurements with a high temporal resolution from different positions on a hillslope at the main recharge area of the Campo de Dalías aquifer, in Sierra de Gádor (Almería, SE Spain). Soil properties and surface cover vary depending on the position at the hillslope, and this variability has an important effect on θ. The higher clay content towards the lower position of the hillslope explains the increase of θ downslope at the subsurface horizon throughout the entire period studied. In the surface horizon (0-0.1 m), θ patterns coincide with those found at the subsurface horizon (0.1-0.35 m) during dry periods when the main control is also exerted by the higher percentage of clay that increases downslope and limits water depletion through evaporation. However, in wet periods, the wettest regime is found in the surface horizon at the upper position of the hillslope where plant cover, soil organic matter content, available water, unsaturated hydraulic conductivity (Kunsat) and infiltration rates are higher than in the lower positions. The presence of rock outcrops upslope the θ sampling area, acts as runoff sources, and subsurface flow generation between surface and subsurface horizons also may increase the differences between the upper and the lower positions of the hillslope during wet periods. Both rock and soil cracks and fissures act disconnecting surface water fluxes and reducing run-on to the lower position of the hillslope and thus they affect θ pattern as well as groundwater recharge. Understanding how terrain attributes, ground cover and soil factors interact for controlling θ pattern on karst hillslope is crucial to understand water fluxes in the vadose zone and dominant percolation mechanisms which also contribute to estimate groundwater recharge rates. Therefore, understanding of soil moisture dynamics provides very valuable information for designing rational strategies for the use and management of water resources, which is especially urgent in regions where groundwater supports human consume or key economic activities.