## 2 Methodologies
### The Hydrology of a Catchment, Basic Concepts  
The water balance of a catchment area is influenced by the amount and intensity of precipitation, evapotranspiration, and runoff, as well as the change in moisture stored in the soil [30]. Precipitation is one of the main sources of water for the catchment; depending on its intensity (mm/h) and its amount (mm), variates the fraction of water that will seep into the ground or runoff into the river or into the basin. However, not all of the atmospheric precipitation reaches the ground surface; in fact, depending on the type of crop and vegetation canopy, some of the precipitation is intercepted and evaporates without ever having reached the ground [31]. Precipitation that does not remain intercepted in the foliage and reaches the soil surface will infiltrate, evaporate, or runoff, depending on climatic conditions (net radiation, air temperature, wind speed, and relative humidity) and soil types and conditions. In fact, depending on the soil texture, the physical properties of the soil change; these include field capacity, saturation capacity, permanent wilting point, and available water capacity [32]. The saturation capacity is reached when all pores are filled with water; however, in this condition, gravity drains some of the water into the lower layers of soil. The amount of water that remains after this drainage is the field capacity of the soil. However, the field capacity does not correspond to the total amount of water available in the soil (available water capacity). In fact, a fraction of the field capacity is bound to solid soil particles by capillary forces and is available neither for evapotranspiration nor for plant nutrition; this amount of water is called the permanent wilting point [33]. Finally, taking away the wilting point, the remaining water percentage of the field capacity is water that is free to move, that is, available water [33]. The texture of the soil, together with the amount of moisture (water) in the soil, characterize the infiltration capacity of the soil. When atmospheric precipitation phenomena occur, if the soil is saturated or if the intensity of precipitation is greater than the infiltration rate, the amount of precipitation that reaches the soil will constitute direct runoff that will feed the river or basin near the catchment. In hydrology, the soil is usually divided into two zones, one called unsaturated, and the other called saturated, separated by an imaginary line called the water table. The water stored in the unsaturated zone is called soil water [34], and this zone is further divided into two zones: the rooting zone and the intermediate zone. The saturated zone, on the other hand, lies below the unsaturated zone and is bounded by two different layers: an upper layer that is the capillary fringe (located at the water table level) and an impermeable bottom that is the bedrock [31]. Precipitation contributes to runoff when its intensity or duration is such that it exceeds the infiltration capacity of the soil and thus surface runoff, also called direct runoff, results. Another water contribution to a catchment also comes from indirect runoff, that is, some of the water that infiltrates into the soil will percolate into the lower layers of the soil until it reaches the saturated zone, going to form groundwater. Some of this water will go into the catchment, constituting indirect runoff. In this work, the rooting zone is considered, since it constitutes a large part of the unsaturated zone, and as such allows for modelling and consideration of fundamental aspects of soil physics that influence evapotranspiration and direct runoff [35]. The SMC data for the root zone are taken from [29], where they refer to a soil depth ranging from the surface to 100 cm. The saturated soil zone, and thus the indirect runoff are ignored in this study. This, however, is a conservative consideration, being that indirect runoff would constitute more water available for hydropower generation.