Efecto de la inclinación y pedregosidad sobre el reparto del agua de lluvia, su cuantificación y aplicación al estudio de la vegetación en zonas áridas

For the correct distribution of vegetation in an area, it is essential to manage accurate information on the variables that condition information. Most likely, the parameter that determines further the type of vegetation that can grow in one place is the amount of water available to plants. In this sense, and leaving aside the formations linked to watercourses, lakes, etc., the main source of this resource is the rain. Thus, the study of the distribution of vegetation in any territory is closely related to the analysis of rainfall it receives. To know the amount of rainwater that receives a zone always uses data provided by meteorological stations located in the same. The data collected by these stations are applied to a hypothetical, uniform and flat surface. This information is accurate enough when the scale at which it works is small (1: 100,000, 1: 50,000), but when it requires greater detail, especially in arid areas where the vegetation structure is open and the soil directly receives much of the rainfall, soil conditions exist that determine the distribution of rainwater and therefore access to this resource plants. Two of these variables, perhaps the most important are the inclination and the presence of rocky outcrops or stoniness on the floor.

In short, our job is to propose different mathematical models that allow to know the actual amount of water available to plants, we call A. This value is obtained from rainfall data (P), relating to the sloping terrain and the percentage of it occupied by rocks (af). The relationship between precipitation and tilt gives us a value we call the real precipitation (P’), which is lower the greater the inclination, since the amount of rainfall should be distributed over a larger area. The amount of rocky outcrop increases runoff, accumulating water in the earthy areas between the rocks, so a higher percentage of stoniness in soil involves an accumulation of water in the surrounding soil. Thus a model that allows both better explain the distribution of vegetation in arid areas and on large scales (: 25.000 or higher 1) is provided.

To test the model and test its usefulness, it has made a study of it in different localities in arid areas of the island of Gran Canaria, one of the Canary Islands. On this island 14 towns located in arid environments, with precipitation always less than 200 mm/m² were chosen. Among these locations, with similar climatic conditions, there is a very important plant diversity. Most are occupied by a crasicaule very open scrub dominated by Euphorbia balsamífera, called tabaibal de tabaiba dulce, typical of the most barren areas of the Canary Islands, and considered the potential of its arid and hyper-arid vegetation areas. But other situations are occupied by a lush vegetation: the cardonal, almost totally enclosed high scrub, dominated by Euphorbia canariensis, the cardón; and even formations characterized by the presence of a undertree thicket where different woody species such as Olea cerasiformis, wild olive, Juniperus turbinata subsp. canariensis, the sabina, and even the Canary Island pine, Pinus canariensis. These same plant formations appear as potential in areas with higher rainfall, so its presence in these arid areas should be related to some variable affecting the distribution of water resources.

For each of these locations was made calculating the amount of water available to plants, obtaining results that meet the alleged contradiction to find different types of vegetation, with different water requirements in the same climatic zone. Thus it is improving the proposed model provides when it comes to study how vegetation is distributed in arid and territories detail scale is checked.

It is clear that as we decrease the scale of the study of the distribution of vegetation in any territory, it is essential to increase the parameters analyzed, especially if it comes to analyzing the situation of vegetation in arid, where the structure of vegetation and seasonality of rainfall make the characteristics of the substrate affect significantly to the presence of vegetation way. Given the scarcity of water resources in these ecosystems, any aspect influencing the availability of water for plants will be of great importance for understanding distribution.