Modeling of vegetation-erosion dynamics in watershed systems

Vegetation and erosion are a pair of competing and interactive factors that affect the quality of watershed ecosystems. The objective of this study is to develop an innovative approach for conceptualizing and simulating the vegetation-erosion dynamics. Differential equations of vegetation-erosion dynamics have been developed to describe the relevant vegetation processes, with the relevant solution methods being provided. Based on the developed model, a vegetation-erosion chart can be produced for predicting the tendencies of vegetation and erosion under different land-use conditions. Thus decision supports in terms of desired measures to improve the system conditions can be provided. In general, vegetation of a watershed may exist in three states, including (1) vegetation-developing and erosion-reducing; (2) vegetation-deteriorating and erosion-increasing; and (3) transitional state between states (1) and (2). Humans may change a watershed system from one state into another. The effort needed for such a change depends on the distance between the present position and the destination one as shown on the vegetation-erosion chart. The developed model has been applied to three regions, including the Xiaojiang, Heishui, and Shengou Watersheds in China. The results demonstrate that the proposed vegetation -erosion dynamics is a powerful tool for simulating and predicting vegetation evolutions in the watersheds. Generally, reforestation and erosion-control measures would improve vegetation coverage slowly in the first 10 years, but become much faster in the second 10 years; this implies that a long-term strategy is needed. The results also indicate that, for revegetating hilly areas, erosion control is critical; merely planting trees and shrubs is insufficient for greening the exposed land.