Analysis Of Temporal And Spatial Variation Characteristics Of Reference Crop Evapotranspiration And Exploration Of Suitable Water-saving Technology Models In Beijingtianjin-Hebei Region

The Beijing-Tianjin-Hebei region,located in the North China Plain,is the"capital economic circle"and one of the most important agricultural production bases of China.In recent years,with the increase demand of water for industrial development and live,the available water for irrigation shows a downward trend,resulting in the rapid decline of groundwater level and many ecological and environmental problems in this area.Appling appropriate measures,such as crop planting structure adjustment and new rainwater collection measures,can not only effectively improve agricultural water supplication in Beijing-Tianjin-Hebei region,but also improve the agricultural water use efficiency and economic benefits in Beijing-Tianjin-Hebei region.To improve the agricultural water use efficiency,some methods,i.e.,the climate tendency rate,Mann-Kendall trend test,Morlet wavelet,Pearson correlation coefficient,and inverse distance weight difference method,were applied to analysis the temporal and spatial variation of reference crop evapotranspiration（ET₀）and its impacting factors in Beijing-Tianjin-Hebei region from 1990 to 2019.Based on the analysis of ET₀,at field planting,the NSGA-II multi-objective genetic algorithm with target of water use efficiency（WUE）and economic benefits was used to optimize crops planting structure in Beijing-Tianjin-Hebei region.For protected agriculture,this paper puts forward the technology of membrane rainwater collection systerm combined with water and fertilizer integration（TMRCS）.The feasibility of TMRCS was further evaluzted according economic benefits,social and ecological benefits.The main results are as follows:（1）For the temporal distribution of ET₀,the average annual ET₀in recent 30 years showed a downward trend,and the decline rate was-3.07 mm per decade.The interannual highest value of ET₀ was 1010.80 mm in 1993 and the lowest value was 896.24 mm in 1990.The results of M-K trend test method show that the interannual ET₀in Beijing-Tianjin-Hebei region mutated in 2016,with a decrease of about 9.71 mm.There are 22～28 years main oscillation periods in the evolution process of ET₀from 1990 to 2019.There are three obvious periods in the evolution process of ET₀,which correspond to the time scales of 10,22,and 28years.The fluctuations of these three periods control the main variation characteristics of ET₀in the whole-time domain.In terms of spatial distribution,the high-value areas of interannual ET₀ are mainly distributed in Southern Hebei,southeast Beijing,and central and southern Tianjin,while the values in northern and northwest Beijing-Tianjin-Hebei region are relatively lower.（2）For the influencing factors of reference crop evapotranspiration,the multi-year average ET₀ in Beijing-Tianjin-Hebei region is positively correlated with the multi-year average temperature（T）,wind speed（WS）,and sunshine hours（SSD）,and negatively correlated with the relative humidity（RH）.The direct path coefficients of temperature,wind speed,and ET₀ are the highest,reaching 0.63 and 0.42 respectively.The results of path analysis show that WS and T are the dominant meteorological factors affecting the change of ET₀in Beijing-Tianjin-Hebei region.The comprehensive influence degree of SSD and RH is not high,and the contribution rate of each meteorological factor is WS>T>RH>SSD from high to low.（3）For adjustment of planting structure:after adjustment,the WUE and economic benefits of Beijing increased by 0.02～0.03 kg/m³and 597～189000 Chinese Yuan（CNY）,respectively.The planting areas of rice,soybean,peanut,and cotton should be reduced to 5.36～4.83 hm²,22.05～70.57 hm²,18.95～61.1 hm² and 0.04～0.26 hm²,respectively.The planting area of corn increased by 127.52～39.03 hm².After the adjustment of planting structure,the WUE and economic benefits of Tianjin increased by 0.03～0.05 kg/m³and18.05～22.135 million CNY,respectively.The main adjustment plan in this area is to appropriately increase the rice planting area of about 997.59～1258.25 hm².Meanwhile,the planting areas of corn,soybean,peanut,and cotton were appropriately reduced by 156.73～230.07 hm²,40.1～140.5 hm²,15.47～58.51 hm²,and 756.2～855.4 hm²,respectively.After adjustment,the water use efficiency WUE of Hebei province has been improved to a certain extent,but the economic benefits have been reduced at the same time.（4）For the development of new water-saving technologies in protected agriculture,the monthly irrigation guarantee rate in January to June of 2000 was 22.2%～100%,and the monthly irrigation guarantee rate in the January to June of 2010 was 35.4%～100%.In addition,the water collected and stored in each month in the second half year of 2010 can ensure 100%irrigation.The analysis results of irrigation guarantee rate for two consecutive years from 2010 to 2011 show that through continuous rain collection and storage for many years,the integrated technology of water and fertilizer in soft water cellar with plastice film surface rain collection can abtain100%guaranteed irrigation.In addition,compared with surface water and groundwater,the EC value and p H value of rainwater are lower,which is suitable for the application of water and fertilizer integration technology,which helps to reduce the blockage of drip irrigation system and promote the quality and yield of crops.In addition,the rainwater collection cellar has the advantages of low cost,large capacity,long service life,environmental friendliness,and recyclability.Compared with the traditional masonry cement rainwater collection cellar,the cost is reduced by more than 50%,and has higher economic and ecological benefits.