The Theory And Methods Of Complex Irrigation System Water Allocation Based On The Mechanism Of Soil Water Potential Control In Southern China

Water is an indispensable resource for economic and social development.Irrigation in agricultural production is of special significance to ensure the continuous increase of grain production.Due to the shortage of water resources in China,the competition of water allocation among industries is harsh,so it is necessary to develop water-saving agriculture.However,the main problems in the process of irrigation water from the water source to the field in southern China are as follows:the blind pursuit of water saving efficiency and neglect of the impact on the farmland ecosystem;water allocation method and model failure risk assessment method of reservoir and pond-weir joint water supply model based on the reasonable scale in the southern irrigation area to be improved;comprehensive evaluation method of irrigation water efficiency being complex,subjective with untargeted results.Therefore,based on field test monitoring and irrigation data collection,this paper discusses the mechanism of soil water potential control irrigation in upland rice field,and determines the threshold of field water requirement with soil fauna as indicator species.Then the water quality allocation model of the reservoir and pond-weir joint water supply mode based on "1000 ha" irrigation unit is analyzed,and the failure probability and the recurrence period of the mode are analyzed.Finally,the comprehensive evaluation method of irrigation water efficiency is proposed.The main contents and results of this paper are as follows:(1)The response of up land riceyield in southern China to different irrigation condition was analyzed,and the water-production functionof dryland paddy was established.The results of field experiment showed that the effect ofwater stress on the physiological status of paddy was delaying the growth stage and decreasing the yield-related index.The model of water-production function suitable for dryland paddy in study area was Jensen model.Tillering stage,heading to flowering stage andjointing-booting stage are the critical period of dryland paddy production,and sufficient irrigation water should be ensured.The indexes that had significant effects on the yield of dryland paddy were ear length,thousand seedweight,plant height and relative heading period.The water consumption indirectly affected the production by affecting these four indexes.(2)The potential control irrigation mechanismof dryland paddy in southern irrigation area was revealed,and the irrigation control index based on soil water potential was proposed according to the simulation results of AquaCrop model.The results showed that the irrigation water would not significantly reduce dryland paddy production in the range of 173.3m3/mu?194.9m3/mu,and the corresponding lower limit of soil water potential was-32.5kPa,and the soil water potential of-30kPa was recognized as the lower limit of control index of water-saving irrigation for dryland paddy in the region.In the-10kPa control conditions,the water-productivity reached its maximum value,2.85kg/m3,which was higher than the conventional irrigation form.(3)The threshold of field water demand to maintain the farmland biodiversity was discussed based on soil fauna index.The results showed that the relationship between soil fauna and soil basis nutrient concentration was not obvious in the study area.The number of soil fauna in the middle and late stages of paddy growth was significantly correlated with the water level in the drainage ditch.The number and biomass of soil fauna community were significantly correlated with the amount of irrigation water per mu.When the irrigation threshold was 180.6m3/mu?194.9m3/mu,the production of dryland paddy would be ensured,and a good field ecosystem would be formed.(4)The calculation model of water allocation for "1000 ha"-scale reservoir and pond-weir joint water supply model was constructed,and the risk assessment method was proposed for this model.The results showed that the conditional expectation combination model could assist people in decision-making ideas,and was easy to understand and master.The predicted value was quite consist with the measured value;88.9%of the year satisfied the 90%confidence requirement.Nash coefficients,root mean square error and R2 values between model predictions and measured data of the most likely combination of were 0.998,84974.5 and 0.939.The method had high accuracy and could provide the reference for reservoir water supply decision.Plackett copula and No.16 were selected to construct the model risk assessment method.The results show that the risk recurrence period of engineering failure is improved from 4-20 years to 10-222 years when satisfying the irrigation guarantee rate of the study area.Analysis based on three typical type years,i.e.the balance years,dry years,special dry years,was conducted.The prediction based on channel-pond joint mode failure risk with recurrence periods of 4 years,10 years,20 years,showed that with actual water supply capacity of pond-weir,the current channel water supply capacity could basically meet the water demand,and even to a certain extent,could reduce water supply,thereby alleviating water demanding pressure.(5)A new method for comprehensive evaluation of irrigation water efficiency in irrigation area was studied.The results showed that the method of combination evaluation based on cyclic correction addressed the problem that the results of single evaluation method were inconsistent and unpersuasive.The comprehensive ranking considering irrigation region and factors of criterion layers,divided the irrigation region into three categories,which provided more specific advices for water-saving transformation construction.The comprehensive evaluation method based on PCA-Copula could solve the problem that the number of function variables is too many to estimate the parameters.Compared with the results of entropy method and catastrophe theory evaluation method,the difference of results ranking and the correlation proved the feasibility of introducing PCA-Copula evaluation method into irrigation water efficiency evaluation.At the same time,the evaluation results of PCA-Copula were evenly distributed between 0-1,and the maximum difference was 0.622,which was significantly higher than that of entropy method and catastrophe theory evaluation method.The evaluation method based on PCA-Copula was beneficial to distinguish the irrigation water consumption level of different irrigation regions more intuitively.