A. Calvo-Cases

Runoff generation, sediment movement and soil water behaviour on calcareous (limestone) slopes of some mediterranean environments in southeast Spain

Abstract An interpretation of soil hydrology and erosion data obtained from limestone areas in southeast Spain is presented in the framework of recent hypotheses on runoff generation mechanisms in Mediterranean environments. The main objective is to synthesise and harmonise the data in theoretical concepts or behavioural models of hydrological functioning of Mediterranean limestone slopes. For this purpose, data were collected in a typical limestone area, with climatic characteristics ranging from subhumid to semiarid. Several sites were chosen with comparable slopes, vegetation and soils along a transect of 20 km to carry out several research projects. The data discussed in this paper consist of information on water redistribution within the soil profiles obtained from long duration (up to 5 h) rainfall simulation experiments with continuous monitoring of soil moisture at different depths. Runoff and sediment concentration data were obtained (i) from rainfall simulation experiments with different antecedent soil moisture conditions and (ii) as natural results from open Gerlach plots on runoff and sediment movement over 4 years. Three conceptual models of water redistribution during the infiltration process could be observed, two of them imply nonuniform water redistribution within the soil profile due mainly to macropore flow caused by specific soil surface characteristics. At the plot scale, runoff generated in bare patches is mainly Hortonian. In plots with previously wetted soils or soils with high infiltration capacities, the generated runoff implies the saturation of the upper soil. Both types of runoff are discontinuous through time and space. The runoff generation mechanisms at the slope scale have been synthesised into two conceptual models: a Hortonian discontinuous runoff model that takes place in the most degraded slopes or during high intensity rain events and, a mixed runoff generation model in less degraded slopes or in previously wet soils, where infiltration excess runoff as well as saturation excess runoff can happen on the same slope. In both cases, slopes behave as a patchwork of runoff and runon areas, the size of the runoff or runon patches is dependent on the climatological conditions. These control the hydrological disconnection between different parts of the slopes. Hortonian and saturation runoff can both be generated and infiltrated downslope.

Post-fire hydrological response and suspended sediment transport of a terraced Mediterranean catchment

In July 2013, a wildfire severely affected the western part of the island of Mallorca (Spain). During the first three post-fire hydrological years, when the window of disturbance tends to be more open, the hydrological and sediment delivery processes and dynamics were assessed in a representative catchment intensively shaped by terracing that covered 37% of its surface area. A nested approach was applied with two gauging stations (covering 1.2 km2 and 4.8 km2) built in September 2013 that took continuous measurements of rainfall, water and sediment yield. Average Suspended Sediment Concentration (1,503 mg l-1) and the maximum peak (33,618 mg l-1) were two orders of magnitude higher than those obtained in non-burned terraced catchments of Mallorca. This factor may be related to changes in soils and the massive incorporation of ash into the Suspended Sediment flux during the most extreme post-fire event; 50 mm of rainfall in 15 minutes, reaching an erosivity of 2,886 MJ mm ha-1 h-1. Moreover, hysteretic counter-clockwise loops were predominant (60%), probably related to the increased sensitivity of the landscape after wildfire perturbation. Though the study period was average in terms of total annual precipitation (even higher in intensities), minimal runoff (2%) and low sediment yield (6.3 t km-2 y-1) illustrated how the intrinsic characteristics of the catchment, i.e. calcareous soils, terraces and the application of post-fire measures, limited the hydrosedimentary response despite the wildfire impact.

Rainfall timing and runoff: The influence of the criterion for rain event separation

Abstract Rain is not uniform in time and space in semiarid areas and its distribution is very important for the runoff process. Hydrological studies usually divide rainfall into events. However, defining rain events is complicated, and rain characteristics vary depending on how the events are delimited. Choosing a minimum inter-event time (MIT) is a commonly used criterion. Our hypothesis is that there will be an optimal MIT that explains the maximum part of the variance of the runoff, with time to runoff used as a surrogate. The objective is to establish a procedure in order to decide upon this optimal MIT. We developed regressions between time to runoff (T0) and three descriptive variables of rain. Our results show that the optimum MIT is 1 hour, which seems to be the minimum period of time required for water in larger macropores to drain and sufficiently modify the effect of antecedent soil moisture on the runoff generation process. Rain events are classified into three significantly different groups: (1) large and intense rains, (2) light rains on wet soil, and (3) light rains on dry soil. Intense rains produce most of the runoff, but there were significant differences between small events in the runoff generated. Of rain events, 63.75% are single-tip events, and many could be dew.

Badlands in the Tabernas Basin, Betic Chain

The complex badland landscape at Tabernas results from a combination of relief amplitude generated by tectonic uplift since the Pliocene and reactivated several times during the Pleistocene, the properties of the Tortonian sedimentary rocks and a predominantly arid climate. The landscape is dominated by deep incision of the main river systems, which continues in part of the headwater tributaries, and characterized by contrasting slope morphologies and a variety of microecosystems. The Tabernas badlands exhibit a diversity of landforms resulting from the combination of multi-age soil surface components that allow a variety of processes to operate at different rates. These are dominated by rilling and shallow mass movements on south-facing hillslopes. On old surfaces and north-facing hillslopes, where biological components are present, overland flow with variable infiltration capacity and low erosion rates prevail. Incision in the gully bottoms occurs in the most active areas.

Runoff Generation in Badlands

Abstract Runoff processes are of paramount importance in badland systems which act as runoff sources with significant effects on and off-site. Badland surfaces are highly dynamic in space and time, generating a wide variety of interacting factors and processes that determine runoff generation and flow paths. Understanding these runoff generation processes is critical to answering basic issues in badlands, such as water erosion and flash flood generation, and will further increase our ability to predict runoff and its off-site effects under the impact of future climate and land use. A global perspective of the factors and processes and the interactions driving badland runoff response and its variability are described here. The main impacts expected from forecast climate change on badland runoff under the different conditions in which badlands exist are also examined. Furthermore, the main gaps in knowledge of badland responses to rainfall and their impacts under current and global change scenarios are identified.