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Agriculture is the world's largest consumer of water resources, and accurate measurement of crop water footprint (CWF) can provide a scientific basis for evaluating the water use characteristics of agricultural production and guiding water management. The measurement of regional CWF requires large amounts of ground data, and remote sensing provides an effective means to scientifically measure the CWF on a large scale. In this study, we developed an effective means to estimate regional maize CWF based on global evapotranspiration (MOD16 and Global Land Data Assimilation System (GLDAS)‐Noah) products in China. And we also calculated the CWF based on the FAO Penman‐Monteith equation (FAO‐PM) on the site scale in China. We assessed the accuracy of these methods by comparing them against eight eddy‐covariance based flux tower measurements. Links and differences behind the results of the three water footprint calculations were analyzed in terms of the basic principles and characteristics of the calculations. The results showed that the CWF had a similar distribution based on the MOD16 and GLDAS‐Noah, reflecting the influence of regional soil moisture on the CWF. Computational rationale analysis of the three quantitative methods showed that the ETc‐based method from the FAO‐PM model was similar to the MOD16‐ and GLDAS‐Noah‐based CWF in the wetter areas, two remote sensing‐based methods considered the water constraints during soil evaporation and water dissipation during dry and wet canopy conditions, and were closer to reality than the ETc‐based methods in measuring the CWF in arid and semi‐arid regions. Key Points: An effective means based on MOD16 and Global Land Data Assimilation System (GLDAS)‐Noah was developed for simulating regional maize crop water footprint (CWF)The accuracy of the estimated CWF based on FAO‐PM was influenced by the soil moistureThe CWF estimated by MOD16 or GLDAS‐Noah was closer to the reality than that estimated by FAO‐PM, especially in arid and semi‐arid region [ABSTRACT FROM AUTHOR]

The extent and mechanisms by which precipitation extremes affect hydrological processes in the critical zone (CZ) of loess possessing a thick unsaturated zone remains poorly understood. To this end, we here employed a coupled liquid‒vapor‒heat‒airflow STEMMUS (simultaneous transfer of energy, mass, and momentum in unsaturated soil) model to investigate the impact of extreme precipitation, at both event and annual scales, on CZ hydrological processes within a semiarid loess site vegetated by apple trees on China's Loess Plateau. At the event scale, the vapor flux was two orders of magnitude lower than the liquid water flux. However, the thermal vapor flux penetrated to depths of 200 cm, whereas the isothermal liquid water flux only infiltrated to 100 cm during the study period, implying that thermal‐gradient‐driven vapor transfer is an important mechanism for deep‐layer recharge (DLR). At the annual scale, the DLR below 200 cm during extremely wet years was 6.5 times larger than that during extremely dry years. In contrast, extreme changes in climate had only limited impacts on evapotranspiration; the difference between extremely wet and extremely dry years averaged only 35 mm, much less than the difference in precipitation, which averaged 310 mm. However, the extremely dry years showed a higher ratio (0.58) between transpiration and evapotranspiration than did the extremely wet years (0.51). The findings reported here improve our understanding of CZ hydrological processes related to precipitation extremes in semiarid loess regions. Key Points: Thermal‐gradient‐driven vapor transfer is an important mechanism for deep‐layer recharge (DLR) in loessThe DLR below 200 cm during extremely wet years was 6.5 times larger than that during extremely dry yearsExtreme changes in precipitation had only limited impacts on evapotranspiration [ABSTRACT FROM AUTHOR]

Relay strip intercropping of spring wheat and maize is practiced on a large scale in arid regions of northwestern China. In this study, a field experiment was carried out during 2012 and 2013 growing seasons to examine water use and crop coefficient of this system in an arid environment. The experiment comprised three treatments: sole-cropped wheat, sole-cropped maize, and wheat–maize intercropping. The grain yields of both intercropped wheat and maize were enhanced by intercropping. The gross economic profit for wheat–maize intercropping was 16.4% lower than that of sole maize and 95.4% higher than that of sole wheat. The overall land use efficiency was improved by intercropping. The time course of leaf area index in intercropping had the similar trends to those in sole crops with relatively low values, which never exceeded 3.0 m 2 m −2 throughout both growing seasons. Compared to weighted means of the sole-cropping systems, wheat–maize intercropping used 26% and 24% more water in 2012 and 2013, respectively. The water-use efficiency of intercropping was nearly the same as the weighted means of the sole crops. Due to the incomplete groundcover in intercropping plots, more water was consumed as soil evaporation. Averaged over two seasons, the ratio of soil evaporation to actual evapotranspiration was 33.4, 20.7 and 24.1% for intercropping system, sole-cropped wheat and sole-cropped maize, respectively. Crop coefficient ( K c) of sole-cropped wheat was 0.19 ± 0.02, 1.05 ± 0.07, and 0.42 ± 0.09 at the initial, mid and late season in two seasons, respectively. K c value of the sole-cropped maize was 0.22 ± 0.03, 1.10 ± 0.06 and 0.60 ± 0.02 at the initial, mid and late season, respectively. The K c values of the wheat–maize intercropping system varied in 0.21 ± 0.03, 0.89 ± 0.05, and 0.78 ± 0.06 at the initial, middle and late wheat growing season, and in 0.85 ± 0.03 and 0.61 ± 0.01 at the middle and late maize growing season, respectively. Therefore, longer growing season and incomplete ground cover are the main factors that resulted in higher water use for wheat–maize intercropping compared to sole crops. Results of this study can help to improve the irrigation efficiency for the wheat–maize strip intercropping system. [ABSTRACT FROM AUTHOR]

Water use efficiency assessment is one of the core research items in water-saving agriculture. The water footprint (WF) of crop production is a comprehensive indicator reflecting water consumption types, quantities and water use efficiency during crop growth period. Based on the theory of water footprint, this study quantified and evaluated the regional differences of water footprint of wheat at the regional scale in Mainland China. Meanwhile, in order to find the dominant factors causing the regional differences of water footprint of wheat, correlation and path coefficient analysis were used to identify the relationship between water footprint of wheat and its impact factors. The results showed that there were obvious regional differences of water footprint of wheat in Mainland China. The water footprint of wheat was relatively low in the eastern part of Northeast China, most of Huang-huai-hai region, parts of the Lower-Middle Reaches of the Yangtze River and Sichuan province, which was less than 1 m3/kg; while the high values of water footprint of wheat were mainly distributed in Northwest China, the northwest of Inner Mongolia, the south of Lower-Middle Reaches of the Yangtze River and the south of Southwest China, which were more than 1.8 m3/kg. From the perspective of water type, the blue water footprint accounted for 49.02%, while the green water footprint accounted for 50.98% among the total WF of wheat. The green water proportion in wheat water footprint increased gradually from northern regions to southern regions. For instance, the proportion of green water footprint was 18.62% in Beijing, while it was more than 80% in most of southern regions. The regional distinction of green proportion in water footprint was in accordance with the precipitation on distribution. The region with abundant precipitation usually has a high proportion of green water in crop water footprint. There were 9 related factors selected for the attribution analysis: average temperature, relative humidity, growing season precipitation, solar radiation, wind speed, fertilizer consumption, pesticides consumption, agricultural machinery and irrigation share. The statistical analysis revealed that the regional variability of wheat water footprint, besides the crop varieties, was mainly caused by the regional disparities of agricultural production level, such as agricultural inputs and irrigation condition. On the other hands, the solar radiation and precipitation were the major climatic influencing factors causing the regional difference of water footprint of wheat. The indirect path coefficient of water footprint of wheat and its impact factors showed that solar radiation had indirect effects on water footprint through temperature, and precipitation mainly through relative humidity. Meanwhile, the fertilizer consumption, pesticides consumption, agricultural machinery power, irrigation had indirect effect on water footprint of wheat through each other influencing factors. The total influence coefficient analysis showed that the chemical fertilizer consumption, pesticides consumption, agricultural machinery and irrigation were the dominant factors that had an effect on water footprint of wheat. The results can provide information for plan-making of agricultural water management schedule. [ABSTRACT FROM AUTHOR]

ABSTRACT Two distinct approaches can be employed in the classification and divisions of wet/dry climate. The first is the rule-driven strategy with predefined threshold values representing climate division boundaries, which is usually performed by Aridity Index ( AI). The second is an automated method in a data-driven fashion, avoiding the direct specification of classification rules while utilizing some forms of cluster analysis. However, various methods for climate classifications raise issues on their applicability and quality. Therefore, evaluation and comparative studies are needed to handle such issues first, in order to analyze their performance and second, to better understand climate characteristics in a region. This article makes a comprehensive analysis and comparison among five classification methods, including four rule-driven methods based on different categories [Penman-Monteith (PM), Thornthwaite, Holdridge, Sahin's method] and one data-driven method (factor-cluster analysis). With the meteorological data for long-term period (1981-2010), the wet/dry climate divisions were performed for 191 meteorological stations in Northwest China ( NW). The results indicated that the overall climate regimes were in agreement for five classifications, but boundaries of wet/dry climate divisions in a data-driven fashion showed a better consistency with topographic features. PM classifications displayed more arid climate types in NW, while the Thornthwaite approach showed an underestimate in arid environments. All wet/dry climate types corresponding to Holdridge and Sahin's classifications were represented in NW, but Holdridge classification is more easily affected by topography and elevation. Complete comparisons among rule-driven methods are difficult to conduct due to different class definitions and low coincident classes. Class definition of climate types for different rule-driven classifications thus needs further investigation. This article highlights the importance of acknowledging the limitations and advantages of different classification systems as well as the dry and wet climate conditions in NW in hope to provide a reference for a similar geographical region. [ABSTRACT FROM AUTHOR]

China faces the dual challenge of grain production pressure and water scarcity. It is significant to reduce water footprint of grain product ( WFGP, m/t) in irrigated farmland. The focus of grain production and agricultural water use, and the precondition is to determine the WFGP and its composition. This paper estimates the WFGP in irrigated farmland of 31 provinces (including municipalities, autonomous regions) a by collecting actual data of 443 typical irrigation districts in 1998, 2005 and 2010, and analyses its temporal and spatial variation in irrigated farmland of China. The result shows that the WFGP in each province decreases with time except in Jiangxi and Hunan, and the average value of all provinces reduced from 1494 m/t in 1998 to 1243 m/t in 2010. The WFGP decreases faster in more developed municipal cities and major grain production provinces. The annual average WFGP in irrigated farmland is 1339 m/t and the blue and green water account for 63.5 % and 36.5 % of the total, respectively. The WFGP and its composition are significantly different between provinces. Generally, provinces distributed inside and beyond Huang-Huai-Hai Plain, have a higher water productivity, lower WFGP and blue water footprint of grain product, while most provinces located in northwest, northeast, southeast and south China have a higher WFGP and lower proportion of green water in the WFGP as a whole. Portion of the blue water footprint ( BWFGP) is not consumed for crop evapotranspiration ( BWFGP) but conveyance loss (B WFGP). The national averaged BWFGP decreases with time and but still remains up to 466 m/t in 2010, making up 34.8 % of the WFGP. In order to safeguard grain security and ease the water resource pressure, the Chinese government should increase investment and apply advanced technology for developing water-saving agriculture, improve the efficiency of water use and further reduce the WFGP. Considering also the contribution of grain output and the relatively high WFGP, the government should give priority to developing water-saving agriculture in the Northeast of China. [ABSTRACT FROM AUTHOR]

Accurate estimation of reference evapotranspiration (ET) becomes imperative for better managing the more and more limited agricultural water resources. This study examined the feasibility of developing generalized artificial neural network (GANN) models for ET estimation using weather data from four locations representing different climatic patterns. Four GANN models with different combinations of meteorological variables as inputs were examined. The developed models were directly tested with climatic data from other four distinct stations. The results showed that the GANN model with five inputs, maximum temperature, minimum temperature, relative humidity, solar radiation, and wind speed, performed the best, while that considering only maximum temperature and minimum temperature resulted in the lowest accuracy. All the GANN models exhibited high accuracy under both arid and humid conditions. The GANN models were also compared with multivariate linear regression (MLR) models and three conventional methods: Hargreaves, Priestley-Taylor, and Penman equations. All the GANN models showed better performance than the corresponding MLR models. Although Hargreaves and Priestley-Taylor equations performed slightly better than the GANN models considering the same inputs at arid and semiarid stations, they showed worse performance at humid and subhumid stations, and GANN models performed better on average. The results of this study demonstrated the great generalization potential of artificial neural techniques in ET modeling. [ABSTRACT FROM AUTHOR]

The water footprint of crop production (WFP) is among the most comprehensive agricultural water-consumption evaluation tools because it describes the amount, type, and location of water consumed. However, existing literatures on WFP quantification and evaluation are only aimed at a specific spatial level, and currently, there is a lack of quantitative research on the effects of different spatial levels on the variation in WFP. Herein, we calculated the spatial distribution of wheat WFPs in China from 2000 to 2014 at five different spatial levels: crop field, county, river sub-basin, provincial, and large river basin. In addition, we analysed and compared the key driving forces of wheat WFP at these spatial levels, focusing on meteorological and human management factors. Each spatial level had distinct spatial distributions. Human management factors demonstrated more significant effects on the WFP of large river basins, but the meteorological factors were the key drivers at smaller spatial levels. In conclusion, our study quantitatively revealed a non-negligible effect of spatial scale on WFP. Thus, for large river basins, agricultural water resource managers at each spatial level must clarify their different priorities across spatial levels while formulating water-saving and regulation measures. [Display omitted] [ABSTRACT FROM AUTHOR]

Climatic elasticity is a crucial metric to assess the hydrological influence of climate change. Based on the Budyko equation, this study performed an analytical derivation of the climatic elasticity of evapotranspiration (ET). With this derived elasticity, it is possible to quantitatively separate the impacts of precipitation, air temperature, net radiation, relative humidity, and wind speed on ET in a watershed. This method was applied in the Wuding River Watershed (WRW), located in the center of the Yellow River Watershed of China. The estimated rate of change in ET caused by climatic variables is −10.69 mm/decade, which is close to the rate of change in ET (−8.06 mm/decade) derived from observable data. The accurate estimation with the elasticity method demonstrates its reliability. Our analysis shows that ET in the WRW had a significant downward trend, but the ET ratio in the WRW has increased continually over the past 52 years. Decreasing precipitation is the first-order cause for the reduction of ET, and decreasing net radiation is the secondary cause. Weakening wind speed also contributed to this reduction. In contrast, regional warming led to an increase in ET that partly offset the negative contributions from other climatic variables. Moreover, reforestation can affect the energy budget of a watershed by decreasing albedo, compensating for the negative influence of global dimming. The integrated effect from precipitation and temperature can affect the energy budget of a watershed by causing a large fluctuation in winter albedo. [ABSTRACT FROM AUTHOR]

Jointly influenced by natural factors and artificial protection measures in recent years, the vegetation coverage of the Loess Plateau has significantly increased. However, extensive vegetation recovery can result in massive water consumption and a severe soil water deficit, which poses a great threat to the sustainable development of the regional ecological system. Maintaining the balance between precipitation and water consumption is an important foundation of ecological security in the Loess Plateau. Based on this, the present study used the GRACE (Gravity Recovery and Climate Experiment) gravity satellite data to simulate the annual actual water consumption from 2003 to 2014 and to analyze the temporal and spatial evolution of the regional precipitation and the actual evapotranspiration (AET). This study also applied the newly developed rainwater utilization potential index (IRUP) to quantify the sustainability of the water balance in the Loess Plateau. The spatial-temporal patterns of precipitation, potential evapotranspiration, and AET from 2003 to 2014 in the Loess Plateau were all analyzed in this study. Based on the results, the annual average precipitation (AAP) and AET in the entire Loess Plateau had significant increasing trends. The analysis of the spatial distribution reveals that the AET was decreasing from the southeast to the northwest in the Loess Plateau. However, the average values of potential evapotranspiration did not obviously change. Based on the estimated AET result, it was determined that the average IRUP had an increasing trend. The increase in the IRUP is due to an increased rate of precipitation that is statistically higher than that of the AET. Consequently, the Loess Plateau experienced a wetting trend during the period of 2003–2014, especially after the Grain for Green project was implemented. The results in this paper were proven by using three different depths of ERA-Interim (a global atmospheric reanalysis product created by the European Centre for Medium-Range Weather Forecasts) soil water content data from the same period and the observed runoff data from 18 different hydrological sites. Consequently, it seems that the vegetation could maintain a sustainable growth with the implementation of the Grain for Green Project. [ABSTRACT FROM AUTHOR]

Improving crop water productivity is necessary for ensuring food and ecological security. To quantify the water utilization in grain production from multiple perspectives, gross inflow water productivity (WP g ), generalized water productivity (WP u ), evapotranspiration water productivity (WP ET ) and irrigation water productivity (WP I ) were screened in this study. Then, we calculated and analyzed the temporal and spatial variation of the water productivity indices (WPs) in the irrigated land of 31 provinces of China based on the data collected from 459 irrigation districts. The results show that the national values of the four indices were 0.694, 0.860, 1.314, and 1.361 kg/m 3 , and almost all of the provincial WPs increased from 1998 to 2010. The Moran's I analysis shows all of the WP indices were significantly clustered, both globally and locally, in the study years. WPs of the provinces in and around Huang-Huai-Hai Plain are high, while provinces in the northeast, south of the Yangtze River and the northwest of China showed lower water use efficiency. There is a large difference among the four indices in the scientific connotation and numerical performance, and the degree of the spatial difference was WP I > WP ET > WP u > WP g . It is necessary to promote WP in the irrigated land of China, and the provinces in the northeast should be chosen as the key regions to develop water-saving agriculture because of the high production of grain and the low value of WP. Policy decision-making based on the spatial pattern of WP and their relationships among indices for specified regions are also discussed in this paper. [ABSTRACT FROM AUTHOR]

• Intercropped wheat had higher water use efficiency than sole wheat. • Wheat outcompeted maize for soil water during the co-growth period. • Transpiration was higher in wheat border rows than sole wheat, especially during the dry period. • A method for evapotranspiration partitioning in wheat/maize strips was proposed. Climate change will likely intensify water shortages, so more attention should be given to the efficient use of agricultural water in agricultural production, especially under rainfed conditions. Therefore, studying and understanding crop water consumption amounts and patterns is of great importance. However, the amount of water consumed by the component crops in strip intercropping systems has received little attention. To close this knowledge gap, a two-year field experiment of sole wheat (Triticum aestivum L.), sole maize (Zea mays L.), and wheat/maize strip intercropping was carried out under rainfed conditions. Soil water content, sap flow, soil water evaporation, and evaporation of canopy interception were measured both in intercrops and sole crops. Soil water movement between strips, evapotranspiration components, and water use efficiency were calculated. Compared with corresponding sole crops, intercropping increased water consumption of wheat and maize during the growing period of each crop. Intercropped wheat obtained soil water from the adjacent maize strip during the co-growth period, while intercropped maize obtained soil water from the adjacent wheat strip after wheat harvest. The amount of water obtained by maize was much less than the amount of water obtained by wheat. The transpiration of intercropped wheat was greater than that of sole wheat, but the transpiration of intercropped maize was lower than that of sole maize. Compared with sole cropping, intercropping increased the sap flow rate of wheat in border rows but reduced sap flow rate of maize in border rows, and this phenomenon was pronounced in the case of drought. After wheat harvest, soil water evaporation from the intercropped wheat strip was less than that from sole wheat, and evaporation from the intercropped maize strip was significantly higher than that of sole maize. The two-year average water use efficiency of intercropped wheat was 32% more than that of sole wheat, and water use efficiency of intercropped maize was 10% less than that of sole maize. These results can improve the understanding of water consumption in wheat and maize of intercropping from the point of evapotranspiration partitioning, and provide implications for water management of intercropping system under rainfed conditions. [ABSTRACT FROM AUTHOR]

Soil water and its efficient use are critical to sustainable productivity of rainfed orchards under the context of climate change in water-limited areas. Here, we combined micro-catchments for collecting hillslope runoff, named fish-scale pits, with mulches to examine water infiltration and water consumption of fruit trees using in situ soil moisture monitoring, the micro-lysimeter and sap flow methods via a two-year experiment in a rainfed jujube orchard on China’s Loess Plateau. This experiment included four treatments: fish-scale pit with branch mulching (FB), fish-scale pit with straw mulching (FS), fish-scale pit without mulching (F), and bare land treatment (CK). The results showed that only about 50% of the rainfall infiltrated the soil for CK during the 2014 and 2015 growing seasons. The fish-scale pit without mulching experienced significantly increased rainfall infiltration by 41.38 and 27.30%, respectively, but also increased evaporation by 42.28 and 65.59%, respectively, compared to CK during the two growing seasons. The jujube transpiration significantly increased by 45.64–53.10% over CK, and the evaporation decreased by 42.47–53.50% when fish-scale pits were mulched with branches or straw. Taken together, the results show that the fish-scale pits and mulching combinations efficiently increased rainfall infiltration and jujube evapotranspiration in the experimental jujube orchard. The findings here provide an insight into the field water management for hillslope orchards in water-limited regions. [ABSTRACT FROM AUTHOR]

Loess Plateau has great uncertainty on drought occurrence due to climate change. This paper analyzes the evolution of precipitation, potential evapotranspiration and standardized precipitation evapotranspiration index (SPEI) based on the Coupled Model Inter-comparison Project Phase 5 (CMIP5) data and regional downscaling model (RegCM4.0). Results indicate that, under RCP2.6 Scenario, the precipitation will increase significantly (5% confidence level) at the rate of 16.40 mm/10 a. However, the potential evapotranspiration is showing non-significant decreasing trend at the rate of 2.16 mm/10 a. Moreover, the SPEI will decrease in the south and northernmost area and increase in the central northern area of Loess Plateau. Under RCP8.5 Scenario, the precipitation will increase significantly (5% confidence level) at the rate of 19.12 mm/10 a. The potential evapotranspiration will non-significantly decrease at the rate of 2.16 mm/10 a and the SPEI is showing increasing trend almost in the whole Loess Plateau. Generally, Loess Plateau is becoming wetter in the central part under RCP2.6 Scenario and the wet area will be enlarged to almost the whole plateau under RCP8.5 Scenario. Based on the results, the water resources will increase under global warming, which may alleviate the water scarcity issue in the Loess Plateau. [ABSTRACT FROM AUTHOR]