Optimal vegetation cover in the Horqin Sands of Inner Mongolia: an evaluation of Eagleson's ecohydrological optimality

Abstract:

Vegetation distribution has received much attention in ecohydrological research because of its essential role in terrestrial hydraulic cycling. Although tightly coupled to precipitation and temperature, the real-life vegetation cover pattern under certain climate conditions is hard to be reproduced theoretically. Eagleson proposed an optimality theory on calculating the optimum vegetation cover (M, defined as fraction of total ground surface covered by vegetation) through a solution of simultaneous equations reflecting the balance of water demand and water supply. Two equations were built under a long-term equilibrium state with fixed vegetation species, climate and soil types. One represented water demand which was derived from the potential canopy conductance- the ratio of potential transpiration to potential bare soil evaporation. The other was for water supply composed of information about precipitation and soil conditions. Their joint solution would be the theoretical optimum vegetation cover. Although the optimality theory was proposed as the theoretical basis of ecohydrology, only few works have been carried out to evaluate it.

In this work, we tested Eagleson’s optimality in the Horqin Sand in Inner Mongolia, China, featured by natural vegetation types of grassland and bushes. MODIS datasets were used as basic data to evaluate two vegetation parameters- vegetation cover (M) and leaf area index (LAI). M was conducted from normalized difference vegetation index (NDVI) with temporal resolution of 15 days and spatial resolution of 250 meters (from 2000 to 2012), and was averaged by 0.39 on the whole area during rainy season (May to September).LAI was used with temporal resolution of 4 days and spatial resolution of 1 kilometers (from 2003 to 2012), and mean LAI on the whole area was 0.45 during rainy season. Meteorological data over 50 years (1951-2012) were obtained from three weather stations in this area. Annual mean precipitation was 365.0mm; annual mean air temperature was 6.2℃; annual mean potential evapotranspiration was 715.4mm. Our result showed proximity to the theoretical optimal state, which meant that Eagleson’s ecohydrological optimality was practical in this area.