M. Martínez-Mena

Factors regulating spatial distribution of soil water content in small semiarid catchments

Abstract The factors which control soil moisture patterns in a semiarid area are determined in two different locations: a burnt zone with little vegetation and an unburnt zone with a greater vegetal cover. In the burnt area, the factors affecting the spatial variability of the soil water content are those considered as local controls such as soil texture and slope. These factors are able to explain a significant part of the spatial distribution of soil moisture in this zone independently of the soil moisture state. In the unburnt area, the factors affecting soil moisture were those related with the presence or absence of vegetation in semiarid environments. The upslope contributing area, aspect, soil profile curvature and soil depth best explained the spatial variability of the soil moisture content in the vegetated zone. The actual influence of these factors showed marked seasonal variations due to changes in the physiological activity of the vegetal cover. These factors, which are in part responsible for the distribution of soil moisture, are not commonly integrated in traditional topographic wetness indices. The topographic index of Beven and Kirkby (Beven, K.J., Kirkby, M., 1979. Hydrological Sciences Bulletin 24, 43–69) is extended to account for the interstorm soil water losses. The new indices incorporating the hillslope aspect improve the prediction power of soil water content patterns in semiarid areas.

The role of antecedent soil water content in the runoff response of semiarid catchments: a simulation approach

Abstract The soil water content is recognized as one of many runoff controlling factors in semiarid environments. A simple, physically based distributed model has been developed to study the role of antecedent soil water content in runoff generation in three small catchments in semiarid southeast Spain. The catchments were set up in two different zones: a burnt area with scarce vegetation and unburnt area with a denser plant cover. The infiltration process is determined by the Green–Ampt equation and the initial soil water content is taken into account by means of maps obtained by Conditional Gaussian simulation. The model was run 2700 times with different soil moisture scenarios and design storms. Stochastic sensitivity analysis was used to examine the role of antecedent soil water content in the runoff response. The results showed that the hydrological response after high intensity, low frequency storms is independent of the initial soil water content. On the other hand, the antecedent soil water content is an important factor controlling runoff during medium and low intensity storms, a type of rainstorm that is relatively frequent in semiarid areas. The sensitivity of the runoff response to soil moisture depended on the predominant runoff mechanisms. When infiltration excess overland flow is predominant, as a result of high rain intensities or less permeable soils, the runoff response is more uniform and does not depend on initial soil moisture. Runoff from less intense storms on soils of higher permeability is controlled by the soil water content of the surface soil layers and is more dependent on initial conditions. The role of the initial soil water content is more important in catchments where the presence of plant cover produces a patchwork of runoff and runon sites, whose spatial pattern is highly dependent on the soil water status.

Factors influencing surface runoff generation in a Mediterranean semi-arid environment : Chicamo watershed, SE Spain

A deeper knowledge of the hydrological response of semi-arid Mediterranean watersheds would be useful in the prediction of runoff production for assessing flood risks and planning flood mitigation works. This study was conducted to identify the runoff generation mechanisms and their controlling factors at the hillslope scale in a Mediterranean semi-arid watershed. Four zero-order microcatchments were selected to measure rainfall and runoff for a three-year period. Two groups of soil were differentiated with respect to the hydrological response. The fine textured, poorly permeable soils of low organic carbon content had a greater runoff coefficient (9%) and lower runoff threshold (3·6 mm) than more permeable, coarser textured soils of medium organic carbon content ( 5 mm h−1); and (ii) a saturation-excess overland flow in the less degraded areas with a high organic carbon content (>2%), high infiltrability (>8 mm h−1) and covered by a dense plant cover (>50%).

Influence of vegetal cover on sediment particle size distribution in natural rainfall conditions in a semiarid environment.

Abstract Sediment samples were collected during four years from two plots, one covered by natural vegetation and the other, a “replica” of the same plot, from which the vegetation was removed manually (disturbed plot), in order to evaluate the effects of vegetal cover on sediment particle size distribution. Particle-size was measured using a Coulter LS130 laser diffraction device. The effective size distribution of the sediment was compared with equivalent measurements of the same samples after chemical and mechanical dispersion (ultimate size distribution) to investigate the detachment and transport mechanisms involved in sediment mobilisation. The ratio between the ultimate particle size composition of the transported sediment and that of the parent soil provided a measure of the particle-size selectivity of the transported sediment. The median effective particle was coarser than the median ultimate particle for almost all the events recorded in both plots, confirming that much of the sediment in the runoff includes a substantial proportion of aggregates. Moreover, the texture of the sediment was finer than that of the matrix soil, with a depletion of clay and sand in exchange for an enrichment in silt, indicating the existence of transport selectivity in both plots. The effect of vegetal cover on the size distribution of the sediment was, particularly evident after high intensity rainfall events. An I30 of 40 mm h−1 was found to be the threshold value for changes to be produced in the sediment texture of the disturbed plot, with aggregates being broken down and 10%–20% more sand particles being observed at this value than after events of lower intensity. No differences were found in the natural plot after rainfall of varying intensities. The vegetal cover in the natural plot reduced the energy available for erosion from the rainfall by 50%, and, the energy from the runoff by 75%, hindering the break down of aggregates and the transport of dense particles. Moreover, the presence of vegetation meant the raindrop was the predominant contributor to soil loss in contrast to the disturbed plot where both raindrop and runoff contributed to the erosion.

Organic carbon and nitrogen losses influenced by vegetation removal in a semiarid mediterranean soil

This research was supported with funds from the Commission of the European Communities, EC Contract No ENV4-CT97-0687 MWISED. The experimental plots are included in the RESEL Network, Spain (Red de Estaciones de Seguimiento y Evaluacion de la Erosion Hidrica en el Proyecto LUCDEME).A reduction in plant cover can lead to an increase in the erosionprocesses that diminish soil quality. Any rise in temperature resulting frompredicted climate changes may aggravate this effect, particularly in semiaridMediterranean areas. Bearing this in mind, the capacity of a soil to preserveorganic matter becomes very important if the soil is to maintain its physicaland chemical properties. Soil organic carbon and nitrogen changes wereevaluatedin a non-disturbed (with natural vegetation) and a disturbed (all vegetationmanually clipped to ground level) pine system. Nine years after vegetationremoval significant differences (p < 0.01) were found in the soil organiccarbon content between plots (top 20 cm), but not in totalnitrogen. In the disturbed plot 0.0232 Mg ha−1y−1 of soil organic carbon were lost through erosionand4.30 Mg ha−1 y−1 throughmineralization. In the first 48 months after vegetation removal the soilorganiccarbon content fell from 40.3 to 28.0 g kg−1. Inthe last 60 months of the experiment the amount of organic carbon in the soilfell from 28.0 to 27.7 g kg−1. This result wasmainly attributable to the intense oxidization, which took place during thefirst 60 months, of organic matter linked to the coarse soil mineral fraction.Up to the 72nd month the losses by erosion were a total of 532.7g, which rose to 1284.4 g by the end of theexperiment(108 months). The effect of vegetation removal in a Mediterranean semiarid arealeads to a rapid decline in the amount of organic carbon stored in the soil.Such perturbation is irreversible if left to nature.

Relations between interrill erosion processes and sediment particle size distribution in a semiarid Mediterranean area of SE of Spain

This research was supported by the EC Environment and Climate Research Programme (Contract no. ENV4-CT97-0687).Abstract Erosion and sediment characteristics were measured using simulated rainfall on two cultivated soils of contrasting lithology (Quaternary calcareous colluvium and Tertiary marls) in a semiarid Mediterranean area of SE Spain. Two rainfall intensities, high (56.0±2.4 mm h −1 ) and medium (31.4±1.4 mm h −1 ), were used in order to know the mechanisms involved in each selected condition. For each simulated event, runoff and sediment were sampled at 1-min intervals on a 1-m wide by 2-m long erosion plot. The erosion rate was calculated as the total amount of soil lost divided by the time period of the test. The duration of the test was that needed to reach steady state runoff, an average time of 24.5 min for Quaternary calcareous colluvium and 17.7 min for Tertiary marls. The size distribution of the transported sediment in the field (effective size distribution) was compared with equivalent measurements of the same samples after chemical and mechanical dispersion (ultimate size distribution) to investigate the detachment and transport mechanisms involved in sediment mobilisation. The results showed that the soil type determined the hydrological response, regardless of the rainfall intensity. The erosional response was, however, determined by the rainfall and soil surface characteristics. In Quaternary calcareous colluvial soils, the predominant erosion process depended on the rainfall intensity, with a prevailing detachment-limited condition in high-intensity events and prevailing transport-limited conditions in those events of medium intensity. These differences in the main erosion processes were reflected in the size of the transported material and in the change in sediment size within the storm. Thus, a time-dependent size distribution of the eroded material (decreasing coarse fractions and increasing fine fractions with runoff time) was observed for high-intensity events. In medium-intensity events, on the other hand, the time-independent size distribution of the eroded material indicated transport-limited erosion. Due to the rapid surface crusting on the Tertiary marl soil, no differences in the main erosion processes or in the sediment size distribution occurred for the different rainfall intensities tested. The erosion of marl soils was determined mainly by the limited quantity of available sediment. The effective size of material was a more sensitive parameter than the ultimate size of the sediment to study the way in which the sediment was transported.

Long-term effect of a single application of organic refuse on carbon sequestration and soil physical properties.

Restoration of degraded lands could be a way to reverse soil degradation and desertification in semiarid areas and mitigate greenhouse gases (GHG). Our objective was to evaluate the long-term effects of a single addition of organic refuse on soil physical properties and measure its carbon sequestration potential. In 1988, a set of five plots (87 m(2) each) was established in an open desert-like scrubland (2-4% cover) in Murcia, Spain, to which urban solid refuse (USR) was added in a single treatment at different rates. Soil properties were monitored over a 5-yr period. Sixteen years after the addition, three of the plots were monitored again (P0: control, P1: 13 kg m(-2), P2: 26 kg m(-2) of USR added) to assess the lasting effect of the organic addition on the soil organic carbon (SOC) pools and on the physical characteristics of the soil. The SOC content was higher in P2 (16.4 g kg(-1)) and in P1 (11.8 g kg(-1)) than in P0 (7.9 g kg(-1)). Likewise, aerial biomass increased from 0.18 kg m(-2) in P0 up to 0.27 kg m(-2) in P1 and 0.46 kg m(-2) in P2. This represents a total C sequestration of 9.5 Mg ha(-1) in P2 and 3.4 Mg ha(-1) in P1, most of the sequestered C remaining in the recalcitrant soil pool. Additionally, higher saturated hydraulic conductivity, aggregate stability, and available water content values and lower bulk density values were measured in the restored plots. Clearly, a single addition of organic refuse to the degraded soils to increase the potential for C sequestration was effective.

Erodibility of agricultural soils in the semiarid Mediterranean area of Spain

Erosion mapping is a useful tool for soil conservation planning. The possible correlation between soil type or lithology with soil erodibility has been tested with the aim of facilitating the accomplishment of erosion maps from soil or lithology maps. Soil erodibility (K factor of the universal soil loss equation [USLE]) was determined from 340 samples taken from 4 soil types, on which approximately 95% of agriculture in the semiarid Mediterranean area is located. Calcaric regosols (K = 0.47) showed the highest erodibility and are significantly different from the other soil types. It was also possible to differentiate between calcic xerosols (K = 0.41) and petrocalcic xerosols (K = 0.37). No significant differences were established between the other soil types. The statistical analysis of the soil samples, grouped according to lithology, showed a better discrimination than is evident from those grouped as a function of soil type. It was possible to establish significant differences among all lithologies w...

Exploring short-term leaf-litter decomposition dynamics in a Mediterranean ecosystem: dependence on litter type and site conditions

AimsPlant litter decomposition plays an important role in the storage of soil organic matter in terrestrial ecosystems. Conversion of native vegetation to agricultural lands and subsequent land abandonment can lead to shifts in canopy structure, and consequently influence decomposition dynamics by alterations in soil temperature and moisture conditions, solar radiation exposure, and soil erosion patterns. This study was conducted to assess which parameters were more closely related to short-term decomposition dynamics of two predominant Mediterranean leaf litter types.MethodsUsing the litterbag technique, we incubated leaf litter of Pinus halepensis and Rosmarinus officinalis in two Mediterranean land-uses with different degree of vegetation cover (open forest, abandoned agricultural field).ResultsFresh local litter lost between 20 and 55% of its initial mass throughout the 20-month incubation period. Rosemary litter decomposed faster than pine litter, showing net N immobilization in the early stages of decomposition, in contrast to the net N release exhibited by pine litter. Parameters related to litter quality (N content or C:N) or land-use/site conditions (ash content, an index of soil deposition on litter) were found to explain the cross-site variability in mass loss rates for rosemary and Aleppo pine litter, respectively.ConclusionsThe results from this study suggest that decomposition drivers may differ depending on litter type in this Mediterranean ecosystem. While rosemary litter was degraded mainly by microbial activity, decomposition of pine litter was likely driven primarily by abiotic processes like soil erosion.

Fluvial sedimentary deposits as carbon sinks: organic carbon pools and stabilization mechanisms across a Mediterranean catchment

The role of fluvial sedimentary areas as organic carbon sinks remains largely unquantified. Little is known about mechanisms of organic carbon (OC) stabilization in alluvial sediments in semiarid and subhumid catchments where those mechanisms are quite complex because sediments are often redistributed and exposed to a range of environmental conditions in intermittent and perennial fluvial courses within the same catchment. The main goal of this study was to evaluate the contribution of transport and depositional areas as sources or sinks of CO2 at the catchment scale. We used physical and chemical organic matter fractionation techniques and basal respiration rates in samples representative of the three phases of the erosion process within the catchment: (i) detachment, representing the main sediment sources from forests and agricultural upland soils, as well as fluvial lateral banks; (ii) transport, representing suspended load and bedload in the main channel; and (iii) depositional areas along the channel, downstream in alluvial wedges, and in the reservoir at the outlet of the catchment, representative of mediumand long-term residence deposits, respectively. Our results show that most of the sediments transported and deposited downstream come from agricultural upland soils and fluvial lateral bank sources, where the physicochemical protection of OC is much lower than that of the forest soils, which are less sensitive to erosion. The protection of OC in forest soils and alluvial wedges (medium-term depositional areas) was mainly driven by physical protection (OC within aggregates), while chemical protection of OC (OC adhesion to soil mineral particles) was observed in the fluvial lateral banks. However, in the remaining sediment sources, in sediments during transport, and after deposition in the reservoir (long-term deposit), both mechanisms are equally relevant. Mineralization of the most labile OC (the intraaggregate particulate organic matter (Mpom) was predominant during transport. Aggregate formation and OC accumulation, mainly associated with macroaggregates and occluded microaggregates within macroaggregates, were predominant in the upper layer of depositional areas. However, OC was highly protected and stabilized at the deeper layers, mainly in the long-term deposits (reservoir), being even more protected than the OC from the most eroding sources (agricultural soils and fluvial lateral banks). Altogether our results show that both mediumand long-term depositional areas can play an important role in erosive areas within catchments, compensating for OC losses from the eroded sources and functioning as C sinks.