Effects of maquis clearing on the properties of the soil and on the near-surface hydrological processes in a semi-arid Mediterranean environment

Mediterranean environment have been cleared in recent decades. There is little information on what effect this has on the hydrology of the soil. We compared the hydraulic properties of the soil and the subsurface hydrological dynamics on two adjacent sites on a hillslope. One site was covered with maquis, the other with grass. The grass started to grow some 10 years ago, after the maquis had been cleared and the soil had been ploughed. Our study found that the hydraulic properties and the hydrological dynamics of the maquis and the grassed soil differed greatly. The grassed soil had less organic matter and higher apparent density than did the soil covered in maquis. Moreover, the maquis soil retained more water than the grassed soil in the tension range from saturation to 50 cm of water. Infiltration tests performed in summer and in winter indicated that the field saturated hydraulic conductivity (Kfs) of the maquis soil was higher than that of the grassy soil. However the data showed that the Kfs of the two soils changed with the season. In the maquis soil the Kfs increased from summer to winter. This was assumed to be due to water flowing more efficiently through wet soil. By contrast, in the grassy soil the Kfs decreased from summer to winter. This was because the desiccation cracks closed in the wet soil. As result, the influence of the land use change was clear from the Kfs measurements in winter, but less so from those in the summer. Changes in land use altered the dynamics of the infiltration, subsurface drainage and soil water storage of the soil The maquis soil profile never saturated completely, and only short-lived, event based perched water tables were observed. By contrast, soil saturation and a shallow water table were observed in the grass covered site throughout the wet season. The differences were assumed to be due to the high canopy interception of the maquis cover, and to the macropores in the grassed soil being destroyed after the maquis had been cleared and the soil ploughed. The results of this work are helpful for predicting the changes in the hydraulic properties of the soil and in the near-surface hydrological processes in similar Mediterranean environments where the natural vegetation has been cleared. These changes must be taken into consideration when developing rainfall-runoff models for flood forecasting and water yield evaluation.</br

Runoff generation processes in a Mediterranean research catchment (Sardinia)

In recent decades the hydrological community has increasingly improved its understanding of the runoff generation in river basins. Since Horton (1933), numerous studies have investigated these mechanisms at the plot, hillslope and catchment scale (e.g.: Betson, 1964; Dunne and Black, 1970; Pilgrim et al., 1978; Kirkby, 1978; Mosley, 1979; Beven, 1989; Anderson and Burt, 1991). The primary processes that have been observed and described to explain runoff generation in a catchment area are: (1) Hortonian Overland Flow (HOF), which occurs when rainfall intensity exceeds the infiltration capacity of the soil; (2) Saturation Overland Flow (SOF), which occurs when the storage capacity of the soil is exceeded and defines the concept of contributing saturated areas, which expand as rainfall volume increases; Saturated Subsurface Flow (SSF), which occurs when the water in the soil flows along lateral paths and thus contributes to streamflow as return flow from the groundwater aquifer.</br

Comparing physical quality of tilled and no-tilled soils in an almond orchard in southern Italy

No-tillage (NT) is an alternative way of reducing costs and lessening the burden of working the land, but in essence it is a method of sustainable land use in dryland cropping systems. The physical quality of the soil is the fundamental factor that defines the sustainability of agro-ecosystems, and its evaluation can be obtained using both capacitive and dynamic indicators. The main objectives of this study were: i) to assess the physical quality of the soil in an almond orchard where long-term different soil tillage systems and weed control methods, such as NT with chemical control and surface tillage (ST), were used; and ii) to compare the indicators under consideration with the proposed reference values, using the information gathered to evaluate the effects of NT and ST. The following physical properties were determined: bulk density, air capacity, macroporosity, plant available water capacity, relative field capacity, Dexter's index, field saturated hydraulic conductivity, as well as the location (modal, median, and mean pore diameter) and shape (standard deviation, skewness, and kurtosis) parameters which corresponded to the equivalent pore size distribution functions. Our results showed that the physical soil indicators adopted were sufficiently sensitive to identify tillage-induced changes and then to quantify the physical quality of rigid to moderately expansive agricultural soils. After thirty years of NT, a set of capacitive indicators, along with measurements of hydraulic conductivity, used in conjunction with an optimal pore volume distribution and the water release curve, unanimously classified the quality of the studied soil as optimal or near optimal

Subsurface flow and large-scale lateral saturated soil hydraulic conductivity in a Mediterranean hillslope with contrasting land uses

The lateral saturated hydraulic conductivity, Ks,l, is the soil property that mostly governs subsurface flow in hillslopes. Determinations of Ks,lat the hillslope scale are expected to yield valuable information for interpreting and modeling hydrological processes since soil heterogeneities are functionally averaged in this case. However, these data are rare since the experiments are quite difficult and costly. In this investigation, that was carried out in Sardinia (Italy), large-scale determinations of Ks,lwere done in two adjacent hillslopes covered by a Mediterranean maquis and grass, respectively, with the following objectives: i) to evaluate the effect of land use change on Ks,l, and ii) to compare estimates of Ks,lobtained under natural and artificial rainfall conditions. Higher Ks,lvalues were obtained under the maquis than in the grassed soil since the soil macropore network was better connected in the maquis soil. The lateral conductivity increased sharply close to the soil surface. The sharp increase of Ks,lstarted at a larger depth for the maquis soil than the grassed one. The Ks,lvalues estimated during artificial rainfall experiments agreed with those obtained during the natural rainfall periods. For the grassed site, it was possible to detect a stabilization of Ks,lin the upper soil layer, suggesting that flow transport capacity of the soil pore system did not increase indefinitely. This study highlighted the importance of the experimental determination of Ks,lat the hillslope scale for subsurface modeling, and also as a benchmark for developing appropriate sampling methodologies based on near-point estimation of Ks,l

Simulation of ground-water flow in steep basin with shallow surface soil

A coupled ground-water/channel flow distributed model has been developed for continuous simulation in a 123-km2 basin. The aim was to analyze the streamflow generation processes in natural vegetated environments. Finite-difference schemes have been used to solve conservation equations of the 2D saturated subsurface flow and the 1D kinematic surface flow. Because of the high hydraulic conductivity of the surface soil, only the saturation excess mechanism of runoff production has been considered. Parameter sensitivity analysis showed the overriding influence of soil storage capacity and conductivity. A grid discretization &gt;100 m produces a hydraulic conductivity greater than physically meaningful, which considerably increases as the space-grid step increases. Results indicate that the model can satisfactorily simulate the water-flow behavior of the catchment after fitting the three parameters of surface hydraulic conductivity, effective porosity, and evapotranspiration losses. These are done after calculating the conductivity as a function of the height of the water table. The simulation efficiency has varied from 87% in the first 5-year calibration period to 85.8% in the subsequent 5-year validation period

Hydrological processes of a closed catchment-lake system in a semi-arid Mediterranean environment

Data collected in 4 years of field observations were used in conjunction with continuous simulation models to study, at the small-basin scale, the water balance of a closed catchment-lake system in a semi-arid Mediterranean environment. The open water evaporation was computed with the Penman equation, using the data set collected in the middle of the lake. The surface runoff was partly measured at the main tributary and partly simulated using a distributed, catchment, hydrological model, calibrated with the observed discharge. The simplified structure of the developed modelling mainly concerns soil moisture dynamics and bedrock hydraulics, whereas the flow components are physically based. The calibration produced high efficiency coefficients and showed that surface runoff is greatly affected by soil water percolation into fractured bedrock. The bedrock reduces the storm-flow peaks and the interflow and has important multi-year effects on the annual runoff coefficients. The net subsurface outflow from the lake was calculated as the residual of the lake water balance. It was almost constant in the dry seasons and increased in the wet seasons, because of the moistening of the unsaturated soil. During the years of observation, rainfall 30% higher than average caused abundant runoff and a continuous rise in the lake water levels. The analysis allows to predict that, in years with lower than the average rainfall, runoff will be drastically reduced and will not be able to compensate for negative balance between precipitation and lake evaporation. Such highly unsteady situations, with great fluctuations in lake levels, are typical of closed catchment-lake systems in the semi-arid Mediterranean environment

Field monitoring and dual permeability modelling of water flow through unsaturated calcareous rocks

Environmental monitoring was carried out in order to investigate the role of topsoil, rock matrix and fractures in controlling the water flow in unsaturated zone of calcareous rocks and in determining the timing and the magnitude of groundwater recharge. A comprehensive dataset of climate data, soil moisture and rock pressure heads was used to evaluate a physically based 1D dual permeability model, in order to simulate the water flow in the vertical profile to the groundwater table. The parameters of water retention and unsaturated hydraulic conductivity curves in soil and rock matrix were estimated using two evaporation experiments in the laboratory and the tension infiltrometer in the field. The other sensitive parameters of the model were calibrated using an optimization procedure based on combinations of randomly sampled parameters. Once the model had been calibrated, it was used to obtain insights into the hydrological processes through the unsaturated profile, down to the water table. The soil acts as a attenuation layer, determining the timing and the magnitude of drainage fluxes towards the unsaturated rock zone. The low permeability petrocalcic horizon below the soil, which frequently occurs in semi-arid regions, plays a key role in controlling the water exchanges between the soil and the rock. Most water flow in the top of the rock profile occurs through the fractures. This fracture flow becomes less along the profile, as a part of it gradually infiltrates into the unsaturated rock matrix. Fracture water infiltrates entirely into the matrix only when the unsaturated rock zone is very thick, so that in sites with a shallow water table fracture flow may be the dominant groundwater recharge mechanism. The flow through the matrix is continuous over time and at the water table it becomes constant and independent of large seasonal and annual variations in rainfall. The flow through the unsaturated matrix is the dominant groundwater recharge mechanism. In dry summers and in drought years the continuous matrix water flow sustains the recharge, thus maintaining high groundwater levels when more water is extracted. This is important in regions where groundwater from unconfined calcareous aquifers are important water resources. The recharge through the fractures is sensitive to the annual rainfall pattern and also contributes to the groundwater in years when the rainfall is greater

Tidal, seiche and wind dynamics in a small lagoon in the Mediterranean Sea

Environmental monitoring and hydrodynamic studies were carried out to understand the forcing mechanisms that are responsible for the circulation patterns of the several small coastal lagoons of Sardinia, in the center of the Mediterranean sea, Italy. The water level observed in the Calich lagoon showed a free-surface oscillation with a semidiurnal period (tide) and a second order oscillation with higher frequency and lower amplitude (seiche). The wind has a local effect on these oscillations and generates a secondary circulation. Numerical analysis confirmed that the dynamic of the lagoon is mainly controlled by a balance between accelerations, barotropic pressure gradients, and bed and wind stresses, and highlighted that the surface seiche is one of the mechanisms responsible for driving residual currents. The employed 2D-horizontal model of lagoon hydrodynamics, forced with tides only at the basin boundary, reproduced the natural mode oscillation period of 0.5 h of the basin. Besides, our study evaluated the accuracy of the simulation of the seiche and wind effects. The plane 2D model, forced with the measured wind stress above the basin surface, simulated stronger seiches by strength and variability in the wind field, in agreement with the measured seiche and wind data. The 5-min time step of our monitoring was, however, too large to quantify accurately the relationship between seiche-induced residual circulation and wind stress. Simulation results indicate that the wind variations at the time scale of minutes could affect the strength of the seiche oscillation. Nevertheless, this variability had insignificant effect on the frequency, suggesting that the oscillatory response is predominantly linear. This analysis showed that tidal, atmospheric and seiche forcing mechanisms are responsible for the circulation patterns of these small Mediterranean wetland areas, generating water level oscillations each one with specific frequency, but with amplitudes of the same order

Field investigation and modelling of coupled stream discharge and shallow water-table dynamics in a small Mediterranean catchment (Sardinia)

In the semi-arid Mediterranean environment, the rainfall–runoff relationships are complex because of the markedly irregular patterns in rainfall, the seasonal mismatch between evaporation and rainfall, and the spatial heterogeneity in landscape properties. Watersheds often display considerable non-linear threshold behavior, which still make runoff generation an open research question. Our objectives in this context were: to identify the primary processes of runoff generation in a small natural catchment; to test whether a physically based model, which takes into consideration only the primary processes, is able to predict spatially distributed water-table and stream discharge dynamics; and to use the hydrological model to increase our understanding of runoff generation mechanisms. The observed seasonal dynamics of soil moisture, water-table depth, and stream discharge indicated that Hortonian overland-flow was negligible and the main mechanism of runoff generation was saturated subsurface-flow. This gives rise to base-flow, controls the formation of the saturated areas, and contributes to storm-flow together with saturation overland-flow. The distributed model, with a 1D scheme for the kinematic surface-flow, a 2D sub-horizontal scheme for the saturated subsurface-flow, and ignoring the unsaturated flow, performed efficiently in years when runoff volume was high and medium, although there was a smoothing effect on the observed water-table. In dry years, small errors greatly reduced the efficiency of the model. The hydrological model has allowed to relate the runoff generation mechanisms with the land-use. The forested hillslopes, where the calibrated soil conductivity was high, were never saturated, except at the foot of the slopes, where exfiltration of saturated subsurface-flow contributed to storm-flow. Saturation overland-flow was only found near the streams, except when there were storm-flow peaks, when it also occurred on hillslopes used for pasture, where soil conductivity was low. The bedrock–soil percolation, simulated by a threshold mechanism, further increased the non-linearity of the rainfall–runoff processes

Anthropological and Climate Change Effects in the Hydrology of the Po Valley

Water shed hydrology is determined by the state of soil surface. Different degrees of soil covering as well as soil exploitation can cause different processes of weather precipitation division into the watershed area. The anthropical evolution due to the expansion of urban areas is having a significant impact on the hydrological responses within the catchment area. In addition to that, the change in precipitations intensity can affect the formation of flood discharge. Using data collected over a time period of over one century, we have analysed both the changes in soil covering due to anthropical developments throughout the Po Valley \u2013 which have caused a remarkable expansion of urban areas \u2013 as well as any variation in precipitation intensity that may have occurred due to climate change. What can be observed is that the anthropologic effects on the catchment hydrology are very important and not negligible. In fact, the top monthly levels of flood discharge have been getting higher over time, in spite of the fact that the total values for yearly rainfall have been getting lower. Land Use Planning can be therefore considered as an important tool in catchment hydrology. Urbanization diminishes the capacity of soils to absorb precipitation, and as a consequence of that, unless proper \u201ccountermeasures are taken\u201d throughout the catchment area, an increase in discharge volumes and a decrease in hydrological response time all over the area will ensue. Such a concurrence of events would lead to more frequent flood episodes for equal peak flows or, conversely, to higher flows and volumes for equal expected frequency