| Insufficient water resources are an important limiting factor for economic and social development and ecological restoration in northern China.As one of the major types of urban landscape vegetation,turf is a water-intensive form of greenery,usually consuming as much as three times more water than other landscape vegetation.As urban green space continues to expand and climate continues to warm and dry,more irrigation recharge is required to perform and maintain the normal function of turf.Since landscape irrigation can often account for nearly two-thirds of urban summer water use and more than half of annual residential water use,reducing water consumption and improving water use efficiency in turf has become a critical issue to be addressed to maintain sustainable urban water use.Turf evapotranspiration,as one of the main forms of water consumption in urban turfs,includes transpiration from plants and evaporation from soil,and is a key part of determining the water consumption capacity of turf and improving water use efficiency.Zhangjiakou City of Hebei Province,a typical arid-wet transition zone in northern China,is a water shortage area as a result of with low precipitation and high evaporation.Located in the northwest of Beijing,Zhangjiakou City has been listed as an important construction area that either maintains the capital’s water retention function or protects local eco-environments(referred to as"the construction of the two areas"),while the lack of water resources has threatened the implementation of"construction of the two areas".In the past 20 years,due to the accelerated development of urbanization,the urbanization rate increased from 25%in 1996 to 38%in 2017,and the average annual increase of the cultivation area of turf in the main urban areas and counties is 176.31×10~4m~2,which has become a giant challenge for the sustainable use of urban water resources.Hence,the study of evapotranspiration processes in urban green areas and the identification of the main environmental factors affecting evapotranspiration are of great practical significance for the effective management of urban water resources.In this study,we investigated the variation characteristics of turf evapotranspiration at different time scales,analyzed the variation characteristics of reference evapotranspiration,and quantitatively assessed the influence of meteorological and soil moisture factors(e.g.,temperature and radiation)on turf evapotranspiration,in order to provide a scientific basis for turf management on water-saving and sustainable utilization on urban water resources.In this study,based on the meteorological data monitored in real time by the HOBO mini-weather station and the historical meteorological data from the national meteorological station,Penman-Monteith formula was used to calculate the reference evapotranspiration for the past 70years,and the dynamic changes of reference evapotranspiration were analyzed at different time scales to identify the key meteorological factors that affected reference evapotranspiration.This study showed that the reference evapotranspiration in the study area ranged from 1074 mm to 1371mm,with an average of 1245 mm during the turf growth period from 1956 to 2017.The reference evapotranspiration increased continuously in the first half of the year and reached a maximum in June,after which it gradually decreased.The average reference evapotranspiration in spring accounted for 31.3%compared with the level of the total growth period,and such a value reached39.7%in summer,20.0%in autumn,and a minimum of 9.0%in winter.Among the meteorological factors,reference evapotranspiration had the highest correlation with photosynthetic effective radiation(R~2=0.89),followed by temperature(R~2=0.86)and sunshine hours(R~2=0.48)in that order.In this study,the evapotranspiration of turf was measured by the lysimeter method,and the principal component regression analysis was used to establish a principal component regression model to clarify the variation patterns of turf evapotranspiration and its key factors under monthly,daily,and hourly scales based on the actual evapotranspiration of turf measured by the lysimeter,meteorological data,and soil moisture.Finally,the reference evapotranspiration and actual evapotranspiration of turfs were used to calculate crop coefficients of turfs,to analyze the distribution interval of crop coefficients,and to reveal key driving factors.The results showed that:(1)Turf evapotranspiration had typically daily and hourly variations.The hourly dynamics of typical weather usually followed the pattern of"high at noon,low in the morning and evening"with single-peak trend similar to inverted"U",which leaded primarily by different meteorological factors and changes in soil moisture.The dynamic pattern of daily evapotranspiration throughout the year followed the characteristic of"low in winter and high in summer",with a maximum(minimum)in June(February)with an average value of 5.27(0.29)mm/d.The annual average evapotranspiration was 2.63 mm/d,and the total evapotranspiration during the study period was 965.10 mm.The temporal changes in the depletion rate of the soil moisture in soil layers at the depths of 10 cm and20 cm were highly consistent with that in turf evapotranspiration.(2)According to the results of principal component regression analysis,the influence of each factor on the turf evapotranspiration was as follows:photosynthetically active radiation>temperature>soil moisture(10 cm)>humidity>wind speed>soil moisture(20 cm)>soil moisture(30 cm).Finally,the principal component regression model obtained by the backstepping of the principal component regression equation was used to simulate the evapotranspiration on odd days during the study period,and the simulated values were high consistent with the measured values(R~2>0.70),indicating that the proposed model achieved desired simulation.Therefore,the dynamic changes of turf evapotranspiration can be simulated by measuring meteorological factors and soil moisture factors,and thus can effectively avoid environmental impacts on direct estimates of turf evapotranspiration.The crop coefficients of turfs were concentrated in 0-0.3 and 0.7-1.0,which were higher than 1.2 in some cases,and the average during the study period was 0.70,with high(low)values occurring in summer(winter).This study established a model used for projecting increases in the water consumption from urban green space based on the increase rate in urban green space area,the estimates of turf evapotranspiration,and different water-saving systems,which aimed to predict increases in water consumption from urban green space under future different scenarios.This study adopted scenario analyses to estimate the increases in water consumption from urban green space in the next ten years by limiting maintenance level of turfs and the increase rate of urban green space area.The simulation results showed that when the urban green space area increased at the current rate,the water consumption from urban green space in the next ten years would increase by 1.458-1.616 million m~3/year,and the annual domestic water per person in Zhangjiakou is 19.71 m~3.Consequently,the annual increase in water consumption from urban green space was the annual domestic water of74-82 thousand residents;if the urban green space maintenance level was restricted,the water consumption of urban green areas in the next ten years would increase by 0.761-0.853 million m~3/year,which was equivalent to the domestic water of 39-43 thousand residents in Zhangjiakou for one year;when the increase rate of urban green areas was limited,the increases in water consumption from urban green areas would reach 1.162-1.296 million m~3/year,1.312-1.454 million m~3/year,and 1.385-1.535 million m~3/year,respectively,with the increase rate of urban green areas being reduced by 20%,10%,and 5%,respectively,which were equivalent to the annual domestic water consumption of 59-66,66-74,and 70-78 thousand residents in Zhangjiakou,respectively.When the increase of area was accelerated by 5%,10%,and 20%,the increases of water consumption from urban green areas were 1.530-1.696 million m~3/year,1.603-1.777 million m~3/year and 1.753-1.935 million m~3/year,respectively,which was equivalent to the domestic water consumption of 78-86,81-90,and 89-98 thousand residents of Zhangjiakou in one year,respectively.In summary,the atmospheric evaporation capacity of Zhangjiakou is gradually improving,which is extremely disadvantageous for Zhangjiakou city such a semi-arid region.Meanwhile,the water consumption from urban green areas is highly correlated with photosynthetically active radiation and temperature.Additionally,for Zhangjiakou area where the climate is warming and drying,it is necessary to accurately predict the water demand of turfs to achieve water-saving irrigation.In this study,a principal component regression model of local turf evapotranspiration was determined to provide a scientific basis for effective control of turf irrigation.After the success of the Beijing-Zhangjiakou joint bid to host the Winter Olympics,the development of Zhangjiakou has continued to accelerate,which can be supported the fact that urban green area are continuously expanding,but the uncontrolled increase in urban green area will result in a giant challenge on sustainable use of water resources in Zhangjiakou.This study utilized a model to project the increases in water consumption from urban green areas based on the increase rate of urban green areas,the estimated value of turf evapotranspiration,and different water conservation systems,aiming to provide a reference for the sustainable water resources in Zhangjiakou. |
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