Evaluating the effects of different land uses on evaporation and stream flow

Abstract

Studying the effects of different land uses on evaporation and stream flow is a hot and frontier topic in global hydrological research. The occurrence and development of flood disasters are restricted and affected by many factors such as meteorology, hydrology, underlying surfaces and human activities. In recent decades, the changes of evaporation and stream flow caused by land use in the river basin have been more and more important. By calculating the long-term average proportion of all land use types in each catchment during the study period, the effects of six main land use types are evaluated one by one, including forest, grassland, pasture land, cropland, shrub land and wetland.
This thesis mainly uses Budyko framework to analyze large-sample catchments with different land uses in the United States. Totally 200 small catchments (<1000 km2) located almost evenly in the continental United States are selected as the research samples. Budyko framework is established with long-term mean precipitation, potential evaporation and actual evaporation from 1981 to 2013. The difference between evaporation index estimated from data and theoretical evaporation index calculated from theoretical Budyko equation is regarded as the best indicator to evaluate the impacts of different land uses qualitatively. Then, the Budyko framework is divided into five bins according to the dryness index and these watersheds are studied in each bin. Besides, the effects of each land use are quantitatively evaluated by making multiple linear regressions. It also discusses two other methods, Fu's equation and Zhang's equation. The best-fit values of parameter w of the main land types are calculated by nonlinear regression. The best-fit w values in this thesis are compared with previous researches to find out some possible reasons causing the differences. 
From the results in this thesis, forest and shrub land coverage evaporate less and lead to more runoff. However, cropland, grassland and pasture land coverage evaporate more and lead to less runoff. It needs to be emphasized that the results show forest land evaporates more than grassland, which is contradictory with what others found. Some physical and hydrometeorological characteristics of the catchments are the possible reasons to explain the results, including mean slopes, precipitation, root systems and so on. Lastly, the evaluation of the impacts of land use in small watersheds on evaporation and stream flow also provides strategic guidance for future land use planning.