دانشگاه اصفهان جغرافیا و برنامه ریزی محیطی 2008-5362 37 1 2026 03 21 Monitoring and Investigating Changes in Water Use Efficiency (WUE) of Different Climates in Kerman Province پایش و بررسی تغییرات کارایی مصرف آب اقلیم‌های مختلف استان کرمان 23 42 30424 10.22108/gep.2026.146361.1742 FA علی مهرابی دانشیار گروه جغرافیا و برنامه‌ریزی شهری، دانشکده ادبیات و علوم انسانی، دانشگاه شهید باهنر کرمان، کرمان، ایران Journal Article 2025 08 19 Abstract Water Use Efficiency (WUE) is a key indicator for understanding the dynamics of carbon and water cycles across different climatic regimes, which is essential for assessing how ecosystems adapt and respond to complex environmental changes. In this study, Gross Primary Production (GPP) and Evapo-Transpiration (ET) data derived from satellite imagery were used to investigate WUE across all climatic zones—humid, semi-humid, semi-arid, and arid—in Kerman Province from 2004 to 2014. To evaluate the influence of various environmental factors—including temperature, precipitation, solar radiation, vapor pressure deficit, Leaf Area Index (LAI), and soil moisture—the geographic detector method and partial correlation analysis were applied. The results showed that over the study period, GPP and ET increased at average rates of 0.0065 g C/m²/year and 2.23 kg H2 O/m²/year, respectively, while WUE decreased at an average rate of 0.01 g C/kg H₂O/m²/year. Among the environmental factors examined, LAI made the largest contribution to WUE in most climatic regions. Furthermore, precipitation and solar radiation emerged as the most important primary climatic drivers influencing the development of climates in the central humid and semi-humid regions, as well as the central and western humid and semi-humid regions of the province, respectively. Keywords: Water Use Efficiency (WUE), Climate Change, Evapo-Transpiration (ET), Gross Primary Productivity (GPP), Kerman Province. Introduction Water Use Efficiency (WUE) is an indicator that links plant carbon gain to water loss through evapotranspiration, operating within the interconnected plant–soil–atmosphere continuum. At the ecosystem level, WUE is commonly defined as the ratio of Gross Primary Production (GPP) to Evapo-Transpiration (ET), representing the amount of organic carbon sequestered per unit of water lost. This metric offers valuable insights for sustainable ecosystem management aimed at optimizing water resource use. WUE is regulated by vegetation traits—such as stomatal conductance and intercellular CO2 concentration—as well as by various environmental factors. Key environmental drivers affecting plant ecosystems include temperature, precipitation, solar radiation, and vapor pressure deficit. Each of these factors has a critical threshold in relation to WUE and these thresholds vary across ecosystems due to differing local environmental conditions. For instance, an increase in average precipitation generally enhances WUE, whereas excessive precipitation may suppress it. In this study, remotely sensed GPP and ET data were used to estimate the WUE of terrestrial ecosystems in Kerman Province. Partial correlation analysis and the geographic detector method were employed to explore the relationships between WUE and its driving factors. The primary objectives of this study were to investigate the spatiotemporal changes in WUE from 2004 to 2024 and identify the dominant driving factors, along with their spatial characteristics that influenced the adaptation and development of the region's ecosystems. Materials & Methods The climatic drivers analyzed in this study included temperature (TEM), precipitation (PRE), and solar radiation (RAD). Temperature and precipitation datasets were obtained from the WorldClim database consisting of monthly averages covering the period from 2004 to 2024. Solar radiation data were derived from the net Surface Solar Radiation (SSR) product of the European Centre for Medium-Range Weather Forecasts (ERA5). Leaf Area Index (LAI) data were obtained from the MOD17 product of the MODIS sensor, which provided 8-day interval data at a spatial resolution of 500 m, spanning from February 2000 to the present. Vapor Pressure Deficit (VPD) data were derived from the TerraClimate dataset, a comprehensive monthly climate product covering global land areas. Soil Moisture (SM) data were obtained from a global surface soil moisture dataset with a spatial resolution of 500 m, covering the period from 2000 to the present. The geographic detector method was employed to detect spatial heterogeneity across geographical areas and identify its driving factors. This method enabled the detection of interactions between two dependent variables, as well as the magnitude and direction of such interactions. In this study, the geographic detector was used to examine the spatial heterogeneity WUE and identify the underlying driving forces. Research Findings To identify the key drivers influencing GPP, ET, and WUE, a partial correlation analysis was conducted to examine the relationships between these three variables and the six environmental factors. The results indicated that PRE had a significantly stronger influence on GPP, ET, and WUE compared to the other climatic factors and exhibited distinct spatial distribution patterns. Precipitation was positively correlated with GPP, WUE, and ET in the central regions, particularly in humid and semi-humid areas. In contrast, a negative correlation between precipitation and these indices was observed in the southern semi-arid and arid regions. Solar radiation was the second most important factor affecting ET, GPP, and WUE. Spatially, positive correlations between solar radiation and both GPP and ET were mainly distributed in the humid and semi-humid regions of the central and western parts of the province. Conversely, a significant negative correlation between solar radiation and WUE was observed in the southern part of the study area. The influence of solar radiation on WUE was most pronounced in the humid and semi-humid central and eastern regions, where the average correlation exceeded 0.65. The effect of TEM on ET and WUE was relatively weak, except in the humid and semi-humid central and eastern regions, where the average correlation exceeded 0.2. Temperature showed a positive and relatively strong correlation with GPP although a negative correlation was observed in some western semi-arid regions. The responses of GPP, ET, and WUE to LAI were significantly stronger than those to other drivers, indicating an overall strong positive correlation. With the exception of scattered negative correlations in the western and southern arid and semi-arid regions, most areas exhibited significant positive correlations. The spatial patterns of soil surface moisture influence on the three indices indicated a generally low correlation. In contrast, the spatial distributions of the effects of Vapor Pressure Deficit (VPD) on GPP and ET showed a high degree of consistency, with positive correlations prevailing in the southern semi-arid regions. The influence of VPD on WUE was particularly prominent in the western semi-humid region, where a positive correlation dominated. Discussion of Results & Conclusion This study qualitatively and quantitatively assessed the effects of climatic and ecological environmental factors using partial correlation analysis and the geographic detector method. The influences of the various driving factors on Gross Primary Production (GPP), Evapo-Transpiration (ET), and Water Use Efficiency (WUE) exhibited clear spatial distribution patterns. Among all factors, Leaf Area Index (LAI) had the greatest impact on GPP, ET, and WUE. Specifically, LAI was more strongly correlated with GPP because changes in LAI directly alter photosynthetic rates, thereby effectively influencing vegetation productivity. Reduced rainfall resulting from successive droughts—particularly in the arid and semi-arid regions of the province—had a notable impact on changes in ET and GPP. Conversely, more favorable rainfall conditions in the humid and semi-humid regions had positively influenced GPP through their effects on vegetation. However, as the vegetation in these regions was predominantly grassland, the increase in GPP remained relatively limited, which in turn led to a comparatively weak response in WUE. The influence of solar radiation on GPP, ET, and WUE was more pronounced in humid and semi-humid regions, where water resources were more abundant. In these areas, increased solar radiation not only accelerated plant transpiration by providing energy for water vapor evaporation, but also effectively regulated vegetation photosynthetic rates, thereby enhancing GPP. However, because the increase in ET was substantially greater than that in GPP, the net effect on WUE was negatively correlated. In this study, we investigated the spatiotemporal changes in WUE in Kerman Province from 2004 to 2024, as well as the driving factors underlying these changes. The results showed that, across the study area, GPP and ET increased by 75% and 57.3%, respectively. In contrast, WUE decreased across 76.45% of the study area. The effects of 6 environmental drivers—temperature (TEM), precipitation (PRE), solar radiation (RAD), Leaf Area Index (LAI), Vapor Pressure Deficit (VPD), and Soil Surface Moisture (SM)—were analyzed by using partial correlation analysis and the geographic detector method. The results indicated that the increase in LAI was the primary driver of the increases in GPP, ET, and WUE. Moreover, the interaction between LAI and other factors was stronger than that among the other driving factors. The effects of climatic factors also exhibited clear spatial differentiation. Precipitation exerted the greatest influence in the humid and semi-humid central regions of Kerman Province, whereas solar radiation had a stronger impact in the humid and semi-humid central and western regions of the province. The findings of this study offer valuable perspectives for understanding the mechanisms driving changes in WUE and can inform future research on ecosystem development across different climatic regions. کارایی مصرف آب، یک شاخص مهم برای درک مکانیزم چرخه کربن و آب در اقلیم‌های مختلف به حساب می‌آید که برای درک چگونگی سازگاری و توسعه اقلیم‌ها با تغییرات پیچیده محیطی ضروری است. در این راستا از داده‌های تولید ناخالص اولیه و تبخیر و تعرق به‌دست‌آمده از پردازش تصاویر ماهواره‌ای، برای بررسی کارایی مصرف آب در کل اقلیم‌های مناطق مرطوب، نیمه‌مرطوب، نیمه‌خشک و خشک استان کرمان طی سال‌های 1383 تا 1403 استفاده شد. به‌منظور ارزیابی نقش عوامل مختلف محیطی شامل دما، بارش، تابش خورشیدی، کمبود فشار بخار، شاخص سطح برگ و رطوبت خاک، بر روی کارایی مصرف آب از روش آشکارساز جغرافیایی و تحلیل همبستگی جزئی استفاده شد. نتایج به‌دست‌آمده نشان داد که در طول دوره مورد مطالعه، شاخص‌های تولید ناخالص اولیه و تبخیر و تعرق به ترتیب با نرخ متوسط 0065/0 گرم کربن بر متر مربع در سال و 23/2 کیلوگرم آب بر متر مربع در سال افزایش، و کارایی مصرف آب با نرخ متوسط 01/0 گرم کربن بر کیلوگرم آب بر متر مربع در سال کاهش یافته است. از بین عوامل مختلف محیطی، شاخص سطح برگ، بیشترین سهم را در کارایی مصرف آب در غالب مناطق آب‌وهوایی دارد. همچنین عوامل بارش و تابش خورشیدی به ترتیب از مهمترین عوامل تأثیرگذار اولیه آب‌وهوایی در توسعه اقلیم‌های مناطق مرطوب و نیمه‌مرطوب مرکزی و مناطق مرطوب و نیمه‌مرطوب مرکز و غرب استان به حساب می‌آیند.

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