Changes of Reference Evapotranspiration and Its Relationship to Dry/Wet Conditions Based on the Aridity Index in the Songnen Grassland, Northeast China

Qiyun Ma,Jiquan Zhang,Mengmeng Wang,Caiyun Sun,Feng Zhang,Enliang Guo

doi:10.3390/w9050316

Abstract

Reference evapotranspiration (ET0) plays an important role in regional dry/wet conditions. Based on the Food and Agriculture Organization of the United (FAO) Penman-Monteith method and daily climate variables, ET0 was calculated for 21 stations in and around the Songnen Grassland, northeast China, during 1960–2014. The temporal and spatial variations of ET0 and precipitation (P) were analyzed in the annual, seasonal, and growing season (from April to October) time series using the Mann-Kendall test, Sen’s slope estimator, and linear regression coupled with a break trend analysis. A sensitivity analysis was used to detect the key climate parameter contributing to ET0 change. By linear regression analysis on the relationship between ET0, P, and the aridity index (AI), the role of ET0 in determining regional wet/dry conditions was analyzed. Results show a higher ET0 in the southwest and a lower ET0 in the northeast, but P was opposite to that of ET0. Evident decreasing trends of ET0 in the annual, seasonal, and growing season time series were detected in almost the entire region by the trend analysis methods. For the entire region, the decreasing trend of ET0 can be linked to the relative humidity and maximum air temperature. The positive contribution of increasing temperature to ET0 was offset by the effect of the significantly decreasing relative humidity, wind speed, and sunshine duration at the 0.05 level during 1960–2014. In addition, the value of ET0 was higher in drought years and lower in wet years.

Highlights

Climate change is an indisputable fact and may accelerate hydrological cycles and redistribute global water resources [1,2]
Where ET0 is the reference evapotranspiration, ∆ is the slope of the saturation vapor pressure curve at a given air temperature, Rn is the net radiation at the crop surface (MJ/(m2 ·d)), G is the soil heat flux density (MJ/(m2 ·d)), γ is the psychrometric constant, T is the mean daily air temperature at 2 m height (◦ C), U2 is the wind speed at 2 m height (m/s), es is the saturation vapour pressure, ea is the actual vapour pressure, and is the saturation vapour pressure deficit
In the northeast and east, the temperature is comparatively low, because there were some valleys and the stations were located in a relatively high latitude area combined with the cold temperate continental monsoon climate, leading to the lower values of ET0 in these areas; while in the southwest, the topography was relatively flat at lower latitudes (Figure 1), in addition to being far away from the ocean, resulting in the lower relative humidity and the higher values of

Summary

Introduction

Climate change is an indisputable fact and may accelerate hydrological cycles and redistribute global water resources [1,2]. ET0 is one of the vital components of the hydrological cycle and controls the energy and mass exchange between terrestrial ecosystems and the atmosphere [3,4]. It is influenced by many factors including climate factors (e.g., temperature, solar radiation), crop factors (e.g., crop pattern, cropping system), environmental conditions, and water resource management [5]. The observation of trends in these climate factors can reveal the possible impacts of climate change or climate natural variability on the hydrological cycle, identify the spatial and temporal variation, and determine the

Methods

Results

Discussion

Conclusion