In arid and semiarid areas, the interplant spaces are usually covered by physical and biological soil crusts. These crusts, though representing an almost negligible portion of the soil profile, have a number of crucial roles. Soil crusts form the boundary between soil and atmosphere and therefore control gas, water and nutrient exchange into and through soils. Concretely, in the last decade, the study of biological soil crusts (BSCs) (complex communities of cyanobacteria, algae, fungi, lichens, mosses and other microorganisms in intimate association with soil particles) has drawn the attention of a growing number of researchers due to the key role they play in numerous processes in the ecosystems where they appear. Unlike physical crusts, BSCs protect soils against erosion by water and wind, and increase soil fertility by fixing atmospheric C and N, synthesising polysaccharides and reducing nutrient losses by runoff and erosion. Through their influence on numerous properties that affect how water moves though soils such as roughness, porosity, hydrophobicity, cracking, and albedo, BSCs play a key role in water processes, such as infiltration and runoff, evaporation and soil moisture. It is widely known the role of physical crusts in decreasing soil porosity and hydraulic conductivity, thus decreasing infiltration. However, there is controversy regarding the role of BSCs in infiltration and runoff processes. Some studies indicate that BSCs increase infiltration, and consequently, decrease runoff, whereas others have reported that they decrease infiltration and increase runoff or that they have no effect on either of them. In addition, the influence of BSCs on other soil water balance components such as evaporation and soil moisture has hardly been studied and the scarce existing studies also show contradicting results. 

With the aim of enlightening the role that BSCs play in the water balance in semiarid areas, in this thesis it has been analysed the influence of different soil crust types, physical crusts and various developmental stages of BSCs, on key soil water balance components such as infiltration-runoff, evaporation and soil moisture, at plot scale. Furthermore, to better understand how these crusts affect hydrological processes, the influence of the type of crust and developmental stage of the crust on different properties that affect water movement and retention in soils has been analysed. Last, spectral characteristics of the different crust types, as well as of vegetation, have been examined with the aim of developing a spectral classification system for differentiation of these common ground covers in semiarid areas that allows their mapping and the modelling of the effects of the crusted areas on hydrological and erosion processes on larger spatial scales (hillslope and catchment). 

To conduct this research, two areas where BSCs are widespread and that represent key spatial distributions of BSCs in semiarid ecosystems were chosen in the province of Almeria (SE Spain): El Cautivo (in the Tabernas Desert), a badlands catchment with silty-loam textured soils, and Las Amoladeras (in the Cabo de Gata-Níjar Natural Park), a flat area with sandy-loam textured soils. 

Our results show that BSCs increase aggregate stability, water retention capacity, and organic carbon and total nitrogen content compared to physical crusts and, within BSCs, these properties increase in the crust and the underlying soil as the crust is more developed (in terms of greater biomass and later-successional species composition). The increase in soil properties with the presence of BSCs is especially noticeable in the top layer of soil (0.01 m) and decreases with depth (0.01-0.05 m) (Chapter I). 

Through their effect increasing surface roughness and physico-chemical soil properties, BSCs increase infiltration and decrease runoff compared to physical crusts. In general, infiltration increases with greater BSC development (Chapter II). However, there are exceptions to this general pattern that are conditioned by other factors such as the spatial scale under study or the type of rainfall. At small plot sizes (0.25 m2) and after 1h-high intensity simulated rainfall (50 mmh-1), we found that well-developed BSCs such as lichens, generate higher runoff rates than less developed BSCs as cyanobacteria, and similar runoff rates to physical crusts (Chapter II). Thus, at microplot scales and under extreme events, the effect of well-developed BSCs in enhancing infiltration due to their greater roughness can be overcome by their ability to clog soil pores when wet, thus increasing runoff. However, when the influence of BSCs on infiltration and runoff is analysed under natural rain events and at larger spatial scales (1-10 m2), we found that, in low intensity rainfalls, runoff decreases with the cover of well-developed BSCs (lichens) and this effect is higher as the plot size increases (Chapter III). Such decrease in runoff with the presence of well-developed BSCs is due to the microtopography that these crusts confer to soils. Under high intensity rainfalls, BSC cover has no significant effect on runoff yield and the main factor acting to determine runoff generation is rainfall intensity (Chapter III). 

The removal of the crust initially causes infiltration to increase. But this effect diminishes over time as raindrop impact reseals the surface and a new physical crust is formed that increases runoff (Chapter II). Moreover, crust disturbance by trampling but, especially by removal, causes a dramatic increase in erosion (Chapter II). Erosion also depends on the type of BSC. Well-developed crusts as lichens and mosses generate lower erosion rates than less developed crusts as cyanobacteria. 

Regarding the influence of BSCs on soil evaporation, under saturation conditions and warm ambient temperatures, soil water loss is quick in all types of surfaces and no significant differences are found in soils with or without BSCs (Chapter V). However, during long cold wet periods, soil water loss is faster in soils devoid of BSCs than in those covered by them. Thus, BSC-crusted soils maintain more soil moisture at the upper soil layer (0.03 m) than adjacent soils where the BSC has been removed, during wet periods. At deeper soil (0.10 m), soil moisture is similar in both BSC-crusted and uncrusted soils. The removal of the BSC causes a higher decrease in soil moisture in fine-textured soils (Cautivo), where the presence of BSCs has a stronger influence on increasing porosity and infiltration, than in coarse-textured soils (Las Amoladeras). During dry soil periods, soil moisture is similar in soils with or without BSCs (Chapter V). 

Last, a quantitative analysis of spectral characteristics of vegetation, physical crusts and BSC developmental stages has demonstrated the possibility of classifying these common ground covers in semiarid areas based on distinctive spectral features (Chapter VI). The application of the classification system developed to multi and hiperspectral provides the possibility for future mapping of spatial distribution and temporal dynamics of BSCs, which is crucial to incorporating the effects of crusted surfaces in current hydrological and erosion models. 

Summarizing, compared to physical crusts, the presence of BSCs increase physico-chemical properties of underlying soils, especially in the first centimeters of soil, and this enhancement is greater as the BSC is more developed. Due to this increase in soil properties and the higher roughness that BSCs provide to soils, BSCs increase water input by increasing infiltration and soil moisture, and soil moisture, and reduce water output by reducing soil evaporation. Hence, compared to physical crusts, the presence of BSCs and, especially the presence of well-developed BSCs, have an overall positive effect on the local water balance in semiarid ecosystems, in addition to having a major role in protecting soils from erosion.