Temporal and spatial distribution of cropland-population-grain system and pressure index on cropland in Jinghe watershed

Pressure index on cropland is one of the exponents to express the situation of land bearing capacity. The capacity shows different characters within different dimensions. Changes of cropland, population and grain were analyzed in Jinghe watershed. Basing on that, characteristics of the changes of the index in the whole area and that of in 31 counties were analyzed, such as the changes of pressure index on cropland, regional difference and the causes of such that. Results show that: (1) On the whole, the area of cropland in Jinghe watershed takes on sustaining decreased trend and the population takes on sustaining increased. But these don't affect the total output greatly. On the contrary, cropland acreage per capita takes on sustaining increased trend because of the increase of grain yield per hectare. What should note is that the change of grain yield per hectare is great due to the disaster of drought. (2) Minimum cropland acreage per capita in this watershed shows the trend of sustaining decreased. Hereinto, the values of pressure index on cropland reaches near to 1 in 1990, and larger than 1 in 1995 and 1999 because of drought and cropland repossession program respectively, namely, the pressure on cropland increase. In 2005, the index decreased to 0.973. (3) Annual variation and regional distribution of pressure index on cropland is large. For example, in 2005, the number of counties with the values of index exceed of 1 decreased from 21 to 12. The counties bearing larger pressure on cropland distributed mainly in loess hilly region and Liupan Mountain, i.e. the area of west-north of Jinghe watershed area. On the contrary, loess hilly region of middle and lower reaches of Jinghe, alluvial plains regions distribute the units with lower pressure on cropland. (4) Multiple cropping index, population density, the proportion of foodstuff, grain yield per unit area act on the pressure index on cropland, but the force is different for different factors in different area. Based on such analysis above, referring to the development of Jinghe watershed area, it is rational to collocating water and land resources within appropriate dimension adopting systematic method, following some policy and measure, such as reinforce the protection of basic farmland, increase the level of agricultural science and technology. At the same time, Jinghe watershed area should develop ecological agriculture, decrease of quantity of pesticide and chemical fertilizer, control the sustaining increase of population and then improve the quality with great efforts