| The Loess Plateau Gully Region of China is an important grain production base,is also a typically dry-farming region in China.This area is located in the zone of mid-latitude continental monsoon climate with four distinct seasons and low precipitation,and the precipitation is not evenly distributed throughout the year.As a consequence,water shortage has become the biggest constraint of environment and development.Green water is defined as the soil water for plant growth,which will eventually evaporate into the atmosphere,including the water consumption of forests,grasslands,wetlands,and farmlands.It is the primary water source for the soil-plant system in terrestrial ecosystems and plays a vital role in the region.Therefore,further studies are needed for green water hydrological process,formation and transformation mechanism by exploring green water evaluation method under the different cover of vegetation so as to offer the scientific restoration and management model for vegetation in the Loess Plateau Gully Region.This research selected three typical planted forests of Robinia pseudoacacia,Platycladus orientalis,and Pinus tabulaeformis in the Nanxiaohegou basin,a typical small basin in the Loess Plateau Gully Region.Based on the field monitoring of the growing season(May to September)in the Nanxiaohegou basin between 2015 and 2017,this research calculated the green water resources of three typical artificial forests with the water balance approach for closed watershed.A combination of field monitoring,hydrological models,and statistical tests was used to evaluate the applicability of green water evaluation method,to improve dual crop coefficient method and to differentiate productive green water from non-productive green water,based on the hydrological,climatic,and ecological processes in the basin.Furthermore,distributed hydrological model SWAT was used to simulate the green water under the different cover of vegetation in Nanxiaohegou basin,and to predict the varied trend of green water in the future period.This is significant for the improvement of water-use efficiency in terrestrial ecosystems.This research contributes to the understanding of the green water cycle and it provides a theoretical basis for ecological and environmental restoration,water and soil conservation,and water resource management in the Loess Plateau Gully Region.The main conclusions are as follows:(1)The average annual precipitation in Nanxiaohegou basin was 550.46 mm during 1955-2017,presenting a slowly decreasing trend after the mutation point occurred in 1990,and the oscillation were 4a and 20a.The annual average daily temperature was 8.69℃,presenting a significant increasing trend after the mutation point occurred in 1996,and the oscillation were 6a,14a,22a and 28a.The forecast results of future precipitation and temperature showed that the precipitation in spring,summer and autumn was less than the value of reference period,while the precipitation in winter was bigger under RCP4.5 and RCP8.5 scenarios,and the temperature would gradually rise during 2030s-2050s,and there might appear extreme temperature in the future.It was highly possible that there would intensify the drought conditions in the fuuture of spring,summer and autumn.(2)Precipitation was the only source of green water in the studied area.The interannual variability in the total amount of green water during the growing season was low for all three types of planted forests.The trend of green water during the growing season was consistent with the precipitation,with the monthly amount of green water increasing from May to July and decreasing from August to September.The green water of Robinia pseudoacacia was slightly more than Platycladus orientalis and Pinus tabulaeformis.The maximum monthly value during the studied period occurred in August 2017,with the amount of green water for the forests of Robinia pseudoacacia,Platycladus orientalis,and Pinus tabulaeformis at 172.0 mm,167.3 mm,and 165.7 mm,respectively.The green water stored in the forests of Robinia pseudoacacia Platycladus orientalis and Pinus tabulaeformis occured negative,demonstrating greater soil water extraction than recharge,and the Pinus tabulaeformis had relatively large value of the green water stored.The change of monthly non-productive green water was fluctuating,and the monthly productive green water changed in an“A”shape with the month.The amount of non-productive green water for the forests of Robinia pseudoacacia,Platycladus orientalis,and Pinus tabulaeformis at 147.47 mm,146.48 mm,and 133.43 mm,respectively.And the amount of productive green water for the forests of Robinia pseudoacacia,Platycladus orientalis,and Pinus tabulaeformis at 155.67 mm,172.03 mm,and 166.17 mm,respectively.The amount of non-productive green water for the forests of Robinia pseudoacacia,Platycladus orientalis,and Pinus tabulaeformis account for 49%,46%and 45%respectively of green water flue.Because of the large consumption of non-productive green water,the green water flue had more potential to improve the utilize efficiency for the three types of planted forests in Nanxiaohegou basin.(3)1948-Penman method had the strongest applicability and the highest calculation accuracy in the Nanxiaohegou basin by the daily potential evapotranspiration calculation with seven kinds of methods,which could meet the needs of the potential evapotranspiration simulation in the studied area and could be used as a recommended method for simplifying the potential evapotranspiration in the area.Meanwhile,the CRAE model had the best applicability and the highest accuracy for the calculation of green water of the three planted forests in the Nanxiaohegou basin,which could meet the needs of the green water flux simulation in the studied area and could be used as a recommended method for simplifying the green water flux in the area.After parameter calibration for the three planted forests,the CRAE model could meet the further requirements of green water flux simulation for different types of vegetation.The productive and non-productive green water estimated by this model at a daily timescale and the monthly green water were consistent with the corresponding values being derived from water balance approach and monitoring.(4)The variation of leaf-area-index and canopy coverage for three typical planted forests were similar that the increase rate was faster in the early part of the growing season to its maximum value in the middle of the growing season,and then was reduced progressively in the later part of the growing season.The base crop coefficient under the conditions of water stress for the three forests significantly increased from the small value in the early part of the growing season to its maximum value in the middle of the growing season.During the final stage of the growing season,the base crop coefficient under the conditions of water stress decreased slightly The evaporative coefficient was large at the beginning of the growing season,but it dropped with continuing growth of the forest until the final stage of the growing season when it started to increase.The crop coefficient rose at the beginning of the growing season and it remained at a high level during the middle stage,before it started to decline in the final stage of the growing season.The Robinia pseudoacacia forest had the largest crop index,followed by the forests of Platycladus orientalis and Pinus tabulaeformis.Additionally,the variation in productive green water that was caused by the rapid change in precipitation differed between the three planted forests,with the lowest variation in productive green water occurring in the Robinia pseudoacacia forest.This finding indicated that the change in meteorological conditions had the smallest impact on the transpiration of Robinia pseudoacacia forest in the studied area.(5)The variation of soil water moisture in different depth for three typical planted forests were different that the soil water moisture of Robinia pseudoacacia was higher than Platycladus orientalis and Pinus tabulaeformis.The soil water moisture of three typical planted forests became increasingly different in soil depth of 0 cm to 20 cm,40 cm to 60 cm,and 80 cm to 100 cm.The topographic wetness index for the forests of Robinia pseudoacacia,Platycladus orientalis,and Pinus tabulaeformis were 8.17,5.81 and 5.02 respectively.The significant differences of three typical artificial forests become more and more bigger with the increasing soil depth by the increase of soil.The study also indicated that there was a strong correlation between soil moisture and topographic wetness index of three typical artificial forests.The soil water storage of Robinia pseudoacacia and Pinus tabulaeformis coule be compensated to certain extent soil water at the end of the study,but the variation in soil water storage of Platycladus orientalis was negative.The soil water storage deficit degree changed in an "W"shape with the month,and the value was larger in the early part of the growing season.During the following stage of the growing season,the soil water storage deficit degree firstly decreased and then increased.The soil water storage deficit degree of surface soil was between 25%to 30%,and it was relatively stable which the soil water storage deficit degree was about 50%at the soil depth below 40 cm.In the whole growing season,the soil water storage compensation degree was positive at the soil depth of 0 cm to 40 cm.The soil water storage compensation degree of Pinus tabulaeformis was the highest.Therefore,Pinus tabulaeformis had stronger adaptability and higher drought resistance capability in the arid environment of Nanxiaohegou basin.(6)SWAT model was used to simulate the runoff of three typical planted forests in Nanxiaohegou basin,and the SUFI2 algorithm in SWAT-CUP software was used to calibrate the model parameters.According to the results of model adaptability evaluation for three typical planted forests,the Nash-Suttcliffe efficiency coefficient and correlation coefficient of the SWAT model were greater than 0.6,and the relative error were less than 15%.As a consequence,the SWAT model could reproduce the dynamic change of runoff for three typical planted forests in Nanxiaohegou basin.The future bule water in RCP4.5 and RCP8.5 scenarios showed a decreased trend for three typical planted forests,and they had the largest decrease of blue water were respectively 2040s and 2050s in RCP4.5 and RCP8.5 scenarios.The future green water and green water flue showed an increased trend in RCP4.5 and RCP8.5 scenarios,and three typical planted forests had the largest increase of green water and green water flue in 2030s.Robinia pseudoacacia had the largest increase of green water and Platycladus orientalis had the largest increase of green water flue.The future green water storage in RCP4.5 and RCP8.5 scenarios showed a decreased trend for three typical planted forests,and they had the largest decrease of green water storage was 2040s. |
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