Impact of planting structure changes on agricultural water requirement in North China Plain

Evaluating farmland water requirements under different planting structures is an important basis for adjusting the agricultural planting structure and formulating feasible regional irrigation schemes. In order to quantitatively estimate the impact of planting structure changes on agricultural water requirement in the North China Plain (NCP), this paper extracted the main crop types and their planting areas based on MODIS NDVI data and TM/ETM remote sensing images from 2002 and 2012. The water demand of seven main crops in the NCP was calculated based on the crop coefficient method and the influence of planting structure change on crop water requirements in NCP was analyzed. The results showed that firstly:from 2002 to 2012, the main crop planting area decreased by 60.7×10⁴ hectares. Among them, the planting area of the four crops was significantly reduced:winter wheat-summer maize (-4.318×10⁵ hm²), cotton (-3.243×10⁵ hm²), rice (-9.3×10⁴ hm²), and spring corn (-4.9×10⁴ hm²). The crops for which planting area increased were:forest fruit (+1.661×10⁵ hm²) and vegetables (+1.26×10⁵ hm²). Secondly, from 2002 to 2012, the total water demand for major crops in the NCP decreased from 74.27 billion m³ to 69.55 billion m³ (a decrease of 6.37%). Among this, the types of crop with reduced water demand were as follows:winter wheat-summer maize (-3.54 billion m³), cotton (-2.43 billion m³), rice (-0.75 billion m³), and spring maize (-0.21 billion m³). On the other hand, fruit trees and vegetables both caused an increase in water demand 1.12 billion m³ and 1.07 billion m³, respectively. Spatially, crop water demand in the NCP showed an overall decreasing trend, with only the Hebei Plain, northwest Shandong Province, and the coastal plain near the urban periphery displaying increasing water demand. The largest increase in water demand occurred in the coastal plain. Finally, the reduction of crop water demand in the NCP caused by reduction of crop planting area amounted to 4.47 billion m³, while that caused by planting structure adjustments only amounted to 0.25 billion m³. Hence, the reduction of the planting scale is the main reason for the decrease in crop water demand in the NCP.