A vegetation-soil-integrated-model (VSIM) to simulate net primary production at watershed scale was developed to explore the effect of soil water dynamic on the primary production processes in arid and semi-arid in northwest China. The model coupled a soil water dynamic module and a vegetation growth module. The former is a daily time step, multi-horizon and distributed spatial model. The later included a mechanism model of stomatal conductance based on the mechanical character of guard cell, which used to reflect both the influence of soil water potential to stomatal conductance and the stomatal control to net photosynthesis and transpiration processes at leaf scale. Scaling up to canopy and watershed scale through considered the effect of canopy structure and heterogeneity of topography.
The main inputs of the model includes photosynthetic characteristics of main vegetation type, metrological data, soil texture and physical properties, and DEM. The outputs are soil water of 4 soil layers, evaporation, transpiration, runoff, net primary production and biomass of leaf, stem and root. The model was used in Zhifanggou watershed, which located in forest steppe zone and belonged to hilly area of Loess Plateau, and the model validation was tested by field observation data sets and RS data sets. In the modeling experiment, simulations show to provide good approximation with field observation data. The simulated biomass of grass and sub-shrub are better than that of arbor and shrub, and the dynamic of LAI have well coherence with the results calculated by Landsat TM data. The model could reflect the processes of precipitation-runoff at the watershed, and indicate the spatio-temporal changes of soil water content.
Ecological security of Chanchuan watershed in the soft rock area of Middle Yellow River was synthetically evaluated and multi-objective programming of land use was forwarded by using RS and GIS techniques along with systems analysis methods. Moreover, according to the landscape ecology theory, digitalized optimum spatial patterns and rational proportion of land use were obtained through computer-aided adjustment with GIS software to get visible images of land use pattern that guarantees ecological security at Changchuan watershed. The results of comprehensive evaluation on ecological security of land use at Changchuan watershed indicate that measures of soil erosion control, ecological and environmental construction has certainly improved the situation of ecological security of this region during past decades, but the current situation of ecological security was not satisfactory. The results of multi-objective programming of land use pattern based on the ecological security evaluation indicate the optimum land use structure should be 3.7% of woodland, 38.6% of brushwood, 49.4% of grassland and 6.3% of crop land. Their spatial distributions were also patterned in light of requirement of ecological security. The average ESI in this region is leveled at relative secure, figuring at around 0.85.