Spatial Modeling of Maximum Capacity Values of Irrecoverable Rainfall Retention by Forests in a Small Watershed
With hydrologic calculations being performed, the most important factor in forming and estimating rainfall runoff is the actual amount of rain precipitations flowing under the tree canopies of the taiga zone. This is due to the fact that only a portion of rainfall reaching the soil through the forest canopy takes part in river runoff formation. At present, in the understanding of hydrological processes there is no unanimity on methods of estimating rainfall retention by the canopies of different tree species in various natural geographical zones. The existing estimation methods rely on empirical observations of net and gross rainfall rather than on the results of physical modeling of moisture on leaf surfaces in droplet form. The disadvantage of the methods existing in the world is that it is not possible to make a spatial estimation of rainfall captured by canopies. The purpose of this study is mapping of maximum rainfall interception capacity for a small forest watershed. The authors have solved the following problems: 1. In-situ modeling of specific water retention in droplet form on leaf surfaces of tree species of the experimental watershed in the middle taiga subzone has been carried out and values of specific water retention per unit leaf area have been determined. Such experiments have never been conducted in the world before. 2. Indirect methods of estimation of the leaf surface area of tree plants depending on the amount of phytomass, age and height of tree stands have been developed. 3. Mapping of tree stand characteristics (such as species composition, height, canopy cover) has been performed and, together with specific rainfall interception capacity determined during experiments provided the basis for a map of maximum rainfall retention capacity by canopies – i.e. a spatial image of irrecoverable rainfall losses has been obtained.
Rainfall is retained on a canopy in droplet form (droplet size varies from 10.6 to 18.6 mg). Specific water retention (mass per unit leaf surface area) is determined by the leaf (needle) roughness. The overall droplet retention on canopies is determined by leaf surface area and precipitation intensity. The maximum mass of rainfall retention on the canopies of individual deciduous trees reaches 77 kg, (3.0-4.0 mm per canopy projection area) and that of coniferous trees is from 24.8 to 58.0 kg (1.9 to 5.9 mm).
Understanding rainfall loss values in hydrology is of key importance in distributed models of rainfall flood. On the basis of the developed cartographic model of rainfall interception, authors performed hydrograph calculation of a single rainfall flood, using genetic methods for the model watershed of the river Reshetka (Russia, Middle Urals). Taking into account rainfall losses for canopy interception makes it possible to reduce the margin of error in calculations of maximum rainfall flood loss from 126 to 25%.
Keywords : forest hydrology; mapping, GIS-technologies; forest science, rainfall floods, modeling, canopy rainfall interception.