Spatio-Temporal Trend Analysis of Reference Evapotranspiration in Central Luzon, Philippines

Understanding trends in reference evapotranspiration (ETₒ) and their influencing factors is crucial for calculations of irrigation requirements and water management. Meteorological data in Central Luzon (1985–2019) were used to estimate ETₒ using the FAO Penman-Monteith method. Spatial and temporal ETₒ trends were analyzed using the Mann-Kendall test and Sen’s slope estimator. Correlation and sensitivity analyses were conducted to analyze the impact of weather variables on ETₒ. Positive correlations were observed for maximum temperature, solar radiation, and wind speed whereas negative correlations were observed for relative humidity and minimum temperature. In general, ETₒ was statistically dependent and most sensitive to solar radiation, maximum temperature, and relative humidity. ETₒ in the stations surrounded by mountains was consistently lower than the stations in the agricultural areas by 197 and 207 mm for the dry season and annual basis, respectively. The increasing trends with a magnitude of 3.98 mm/yr in annual ETₒ were caused by the rising trend in solar radiation and maximum temperature and a decreasing trend in relative humidity. The decreasing ETₒ trend (–2.6 to –4.63 mm/yr) predominant in the study area was mainly attributed to the decreasing trend of solar radiation and wind speed. Areas with increasing ETₒ trends indicate the need for supplemental irrigation. Decreasing ETₒ trends may indicate climate change, land-use change, or human-related factors.