| Resent years,the issue of food safety has attracted great attention to society at large.The alfalfa(Medicago sativa L.),a kind of high-quality forage,was planting more and more extensively in area at a fast speed.The planting area of alfalfa was increasing quickly,and most of the areas have been equipped with center pivot sprinkler irrigation system(center pivot),especially in arid and semiarid areas.However,facing the problems of the dry climate,scarce precipitation and the lack of water resources for arid and semiarid areas,how to improve the water use efficiency of alfalfa planting area is the critical issue in those areas.The main reasons resulted the declines of water use efficiency in arid and semiarid areas during sprinkler irrigation are wind drift and evaporation losses(WDEL),the water intercepted by plant canopy and the nonuniform water distribution.In order to study the water loss processes and water use efficiency for sprinkler irrigation for alfalfa,we firstly analyzed the water vapor variations for the overhead sprinkler irrigation,then the actual rule of water vapor dynamics was observed in field condition by using a center pivot simulator;basing on the studying results of water vapor dynamics,the generating characteristics and dynamics of WDEL were studied;based on analyzing the changing rules and dynamics of canopy interception during irrigation,the dynamic processes of canopy interception were observed by using a simulated precipitation experiment and a interception collecting device;and according to the survey for practice of alfalfa production used by center pivot,the water distribution charachteristics of overhead sprinkler irrigation were observed,by means of actual field experiments and numerical simulated experiments for two types of common used sprinklers.All the work was aimed to explore the water loss possesses for overhead sprinkler irrigation for alfalfa.The main studying results are as follows:(1)The dynamic variations of water vapor content along with the time duration for the sprinkler irrigation within the irrigated area were analyses,and the dynamic model to describe the variations was developed:the increasing dynamic of water vapor during irrigation could be indicated by an exponential function,and the decreasing dynamic of water vapor after irrigation could be indicated by a Logistic function.The developed model could describe quantificationally the water vapor increments generated by irrigation at any time.And it also could acquire the information of maximum value of water vapor increments,the length for water vapor content within irrigated to restore the level of surrounding atmosphere,and increasing and decreasing rates of water vapor.Furthermore,based on the water vapor dynamic model,the WDEL dynamic model was developed.It can do the functions to describe the dynamics of WDEL’s generation and estimate the value of WDEL.The field experiments were carried out to validate the model under conditions of different distances from sprinkler,and the value of Nash-Sutcliffe model coefficient was larger than 0.934.The parameter analysis for the model showed that the increased water vapor content caused by irrigation were 2.506-6.476 g m-3 and 1.277-3.380 g m-3 for positions where sprinkler located and 2 m away from sprinkler,respectively.A higher vapor pressure deficit(VPD)and temperature(T)and lower relative humidity(RH)could promote the increasing and decreasing rates for water vapor during and after irrigation respectively.And then,functions to estimate values of model parameters were given,with using meteorological factors as variables.It can estimate the dynamic processes of sprinkler irrigation with knowing the meteorological conditions.The observed values of WDEL were from 3.89-35.99 kg,accounting for 0.47%-4.28%of irrigation volumes,and the value was increased with the increasing VPD and T.The rates of WDEL generating were 0.212-1.193 kg m-3 h-1 and 0.083-0.661 kg m-3 h-1 for positions.At last,the estimating model was presented to predict value of WDEL based on variables of VPD and T,and it can be used to estimate value of WDEL in a convenient way.(2)The dynamic variations of canopy interception for the alfalfa canopy under condition of sprinkler irrigation was analyzed,and the dynamic model for alfalfa canopy interception was developed,which can be indicated by an exponential function.The dynamic model can be used to describe the amounts of interception quantificationally at any time duration of irrigation,and to obtain the maximum interception(Im).Further,the study presented a nonlinear estimating model for Im based on variables of plant height(H)and irrigation intensities(R),and it can be used to estimate canopy interception conveniently under field condition.We used a dynamic measuring device to measure the interception dynamics,under different vegetative stages(5 d-45 d)and sprinkling intensities(2.5-40.0 mm h-1).The Nash-Sutcliffe model coefficient reached a value of 0.977.Value of In was 0.29-1.26 mm,accounting for 2.92-12.56%of irrigation volumes.The value of Im was increase with the increase of plant height and sprinkling intensity.The predictive functions for model parameters H and R were presented,so as to predict the dynamics of canopy interception for different vegetative stages and irrigation intensities.At last,we discussed how much time at least should be need to obtain and measure the maximum capacity of canopy interception,and also presented the methodology to determine the time duration according to H,R and experimental error.(3)The water distribution characteristics of two kind sprinklers used for center pivot,the rotating spray plate sprinkler(RSPS)and the fixed spray plate sprinkler(FSPS),was studied.The results of field measurements for the individual sprinkler showed that the RSPS and FSPS possessed different radial water distribution characteristics.For RSPS,the application intensity was decrease as the distance away from sprinkler.While for FSPS,the application intensity was focused over a certain distance away from sprinkler.Based on the probability density function,the model for describing radial water application for RSPS and FSPS was developed,based on the deformed Gaussian function and deformed Lognormal function,respectively.The two developed models were validated by actual field measurement of RSPS and FSPS radial water distribution,with significant fitting determination coefficients of 0.851-0.976 and 0.890-0.998 for RSPS and FSPS respectively under three nozzle diameters of 2.78、4.76 and 6.75 mm.The model can be used to estimate application intensities at any distance away from sprinkler,as well as the maximum application intensities and wetted radii.According to the analyses for model parameters,under different nozzle diameters of 2.78、4.76 and 6.75 mm,for RSPS,the maximum application intensities were 4.83-21.60 mm h-1,which happened 1.45-2.44 m away from sprinkler.And the wetted radii were 5.96-7.62 m.For FSPS,the maximum application intensities were 13.95-39.46 mmh-1,which happened 3.00-4.45 m away from sprinkler.And the wetted radii were 4.00-5.70 m.Among them,each index was increased as the nozzle diameter increased.And the FSPS had a much more application intensity and less wetted radii,which meant a more concentrated water distribution pattern,with comparing to RSPS.Under conditions of field measurements,individual RSPS distributed most water around the sprinkler,whereas individual FSPS distributed most water over a ring-shaped region at the periphery of the sprinkler.Same-nozzle-sprinkler pipe sections of RSPSs distributed most water around the central axes of the pipe sections,and their sprinkling uniformities were 44.7-51.0%,whereas FSPSs distributed most water over both sides of the axes,symmetrically,and less water around the axes,and their sprinkling uniformities were 40.3-58.0%.According to the nozzle configuration model,the sprinkling uniformities of the full circular irrigated areas were 85.8-91.7%and 85.8-86.2%when using RSPSs and FSPSs,respectively,under sprinkler intervals of 1.5,3.0 and 4.5 m respectively.And the uniformities were 3.1%and 6.2%higher using RSPSs than FSPSs with sprinkler intervals of 3.0 and 4.5 m,respectively.RSPS accommodated larger sprinkler intervals and maintained superior sprinkling performance when compared with FSPS.In summary,the current researches suggested the water loss processes and the problems of water use efficiency in terms of water distribution,for overhead sprinkler irrigation for alfalfa.Those developed and validated models,including the water dynamic model,the WDEL dynamic model,the predictive model for WDEL and the model for describing radial water application for individual sprinkler,achieved the objectives to describe above problems concerned with water loss processes and water distribution.Additionally,those models also can be used to estimate the quantities of water loss reliably for an irrigation event. |
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