Study On Energy Balance And Water And Carbon Flux Of Jujube Forest Ecosystem Under Drip Irrigation In Arid Area

Ziziphus jujuba Mill.（jujube）is a tree species that provides both ecological and economic benefits.Jujube trees have been successfully introduced to Xinjiang Uygur Autonomous Region,China over the last two decades,with the planted area reaching 500,000 ha.The increasing area of jujube forests in arid zones has aggravated the challenge of water scarcity.However,few studies have explored the carbon balance of jujube forests during either the reproductive period or entire year.An in-depth study of the processes and energy-driven mechanisms regulating water-carbon flux cycles in jujube forest ecosystems in arid zones is needed to develop approaches for managing the local greenhouse effect,alleviating water scarcity,and improving water use efficiency（WUE）in arid zones.This study conducted continuous flux observations in drip-irrigated jujube forests in the arid zone for two years using an eddy correlation system and auxiliary observation equipment.Missing flux data were interpolated using the R language“REddy Proc”package.The terms of thermal storage in the energy balance were also corrected to analyze the energy balance closure of jujube forests and the trend of each energy component.Management of forest water consumption in arid zones requires estimations of evapotranspiration.This study simulated evapotranspiration using the SIMDual-Kc model and improved S-W model.The relationships between evapotranspiration in jujube orchards and driving factors were analyzed using the structural equation model（SEM）.The R language“REddy Proc”package was used to split net ecosystem CO₂ exchange（NEE）into total ecosystem respiration(R_eco)and total primary productivity（GPP）to analyze the characteristics of carbon flux changes and water use efficiency WUE.The results showed the following.（1）The REddy Proc package could interpolate the data better with the error statistic parameters of b=0.86-0.99,R²=0.86-0.95,d=0.96-0.98,EF=0.84-0.92,and RSR=0.28-0.40.After conducting the R package interpolation,the energy balance closures of the jujube date forests were 73.45%and 73.11%,respectively.R² of the 2-year effective energy（H+LE）and available energy（R_n-G）was 0.97.After adding the thermal storage terms,the energy balance closures increased to 77.17%（by 3.72%）and 75.86%（by 2.75%）,respectively.R² of the effective energy（H+LE）and the available energy（R_n-G-S）was 0.97,respectively.The daily-averaged variation pattern of each energy component was similar during the fertility and dormancy with a single-peaked daily variation according to the variation pattern of the net radiation.The annual energy balance closures of the jujube date forests were 73.45%and 73.11%,respectively.The thermal storage terms had some influences on the improvement in the energy balance closure of jujube date forests.（2）The SIMDual-Kc model was able to provide estimates of evapotranspiration and plant transpiration during the reproductive period that showed good consistency with measured data.When compared against measured data,the model simulations of evapotranspiration obtained determination coefficient（R²）,index of agreement（d）,Nash Sutcliffe Efficiency（EF）,and Standard Deviation Ratio（RSR）goodness-of-fit values of 0.57-0.60,0.86-0.87,0.51-0.55,and 0.67-0.70,respectively,whereas those of simulated plant transpiration were 0.73-0.77,0.91-0.92,0.62-0.64,and 0.60-0.61,respectively.The parameters of the SIMDual-Kc model were:Basalt crop coefficient K_{cb ini}=0.18,K_{cb mid}=0.7,K_{cb end}=0.7,Depth of evaporation layer Z_e=0.15m,Total evaporation water TEW=18mm,Readily evaporable water REW=19mm,and Soil water consumption ratio p=0.5.Model scenario analysis showed that under a constant total irrigation depth,different irrigation frequencies had greater and smaller effects on soil evaporation and plant transpiration,respectively and soil evapotranspiration increased with increasing irrigation frequency,indicating that soil surface water content had a greater effect on soil evapotranspiration.（3）The S-W model shows good performance in simulating evapotranspiration and transpiration in arid areas under parameter value with a₁ of 6.9445,g₀ of 0.0439,b₂ of 4.6041,b₃ of957.1106,achieving root mean square error（RMSE）of 0.62-0.76mm/d,regression coefficient（b）of 0.77-0.88,correlation coefficient（R²）of 0.95-0.97,index of agreement（d）of 0.92-0.94,efficiency of simulation（EF）of 0.70-0.81,RMSE-observations standard deviation ratio（RSR）of0.44-0.55.The daily-scale model simulations were more accurate than the hourly-scale simulations.The model was shown to be vulnerable to weather conditions,and the simulation accuracy for sunny days exceeded that for rainy days.（4）The results of SEM indicated that six factors explain 84%and 89%of observed variations in soil evaporation and transpiration,respectively.The coefficient of influence of plant transpiration of soil evaporation is-0.70.Leaf area index has the highest effect on plant transpiration,with total path coefficient of 0.52,followed by air temperature with total path coefficient of 0.44 and no indirect influence.Net radiation had the highest effect on soil evapotranspiration,followed by soil surface water content,with total path coefficients of 0.51 and 0.49 and direct path coefficients of0.66 and 0.69,respectively.（5）The evapotranspiration of jujube forest showed low variation during the dormant period and high variation during the reproductive period,reaching peaks in July with total annual evapotranspiration of 657.64 mm and 677.40 mm over the two study years,respectively.The NEE,GPP and R_eco of the jujube forest showed obvious seasonal variations,with low variation during the dormant period and drastic changes during the reproductive period.The annual totals of NEE over the two study years were-569.08g CO₂/（m²·a）and-491.96 g CO₂/（m²·a）,respectively,indicating a carbon sink function.In contrast,the annual totals of GPP over the two study years were 1364.86g CO₂/（m²·a）and 1301.19 g CO₂/（m²·a）,respectively,whereas the annual R_eco were 782.46 g CO₂/（m²·a）and 809.22 g CO₂/（m²·a）,respectively.（6）The correlation coefficients representing the relationship between total ecosystem respiration R_eco and soil temperature were highest at a soil depth of 8 cm（R² of 0.72 and 0.67 over the 2,respectively）.The relationship between R_eco and air temperature obtained an R² of 0.72 and0.75 over both years,exceeding the correlation between R_eco and soil temperature.The whole-year WUE values of jujube forest over the two years were 2.08g C/kg H₂O and 1.92g C/kg H₂O,respectively.The WUE of jujube forest during the reproductive period showed fluctuating changes,and the average daily WUE values during the reproductive period were 2.19 and 2.01g C/kg H₂O over the two years,respectively.The WUE of the jujube forest was lower than that of other temperate forest ecosystems,but higher than that of wheat farmland ecosystems.This study demonstrated the energy and water-carbon flux exchange patterns of drip-irrigated jujube forest ecosystems in arid zones.The results of this study can provide a theoretical basis for formulating reasonable drip irrigation systems,irrigation management,and the evaluation of ecological benefits in arid zone jujube forests.