Copula-based risk analysis of agricultural water shortage under natural
precipitation supply condition in semiarid regions
Abstract
Accurate risk forecasting of agricultural water shortage has important
meaning for the prevention and reduction of disasters in regional
agricultural production. The encounter situation of effective
precipitation (Pe) and crop water requirements (ETc) can determine the
regional agricultural water shortage risk under natural precipitation
supply condition. The Guanzhong Plain (GP), divided into Eastern
Guanzhong Plain (EGP) and Western Guanzhong Plain (WGP), is sensitive to
water shortage as a result of the local arid and semi-arid climate.
Based on the daily meteorological data at six representative weather
stations in GP from 1962 to 2016, Pe and ETc, which were marked as Pey,
Pew, Pes and ETcy, ETcw, ETcs over a hydrological year, winter wheat
growth period and summer maize growth period respectively, were computed
and their marginal distributions were examined. Also, the copula
functions were employed to model the joint distribution of Pe and ETc.
The results indicated that the optimal fitted marginal distributions for
Pey, Pew and Pes proved to be Lognormal, GEV and Normal in EGP,
respectively, and Gamma, Log-Gamma and Weibull in WGP, respectively. GEV
correlated all the ETcy, ETcw and ETcs optimally both in EGP and WGP.
The Frank copula was identified as the most suitable model for the joint
modelling of Pe and ETc series. According to the joint probability
distribution, the asynchronous encounter probability of Pe and ETc was
around three or four folds of the synchronous encounter probability in
GP. Further, probabilities of the occurrence of slight, moderate,
severe, and extreme agricultural water shortage under natural
precipitation supply condition were 0.51, 0.20-0.30, 0.04-0.10, 0.01 in
EGP, respectively, while the corresponding values were 0.52, 0.20-0.23,
0.03-0.09, 0.01 in WGP, respectively. Overall, GP confronted serious
agricultural water shortage, and such incompatibility was more serious
in EGP compared with WGP. This study can provide an important guidance
for the adjustment of crop planting structure, the establishment of
irrigation system and the optimal allocation of regional water
resources.