Choice of potential evapotranspiration formulas influences drought assessment: A case study in China

Abstract

Droughts reflect the extent of water deficit on the land surface, and can be quantified by various indices. In current study, potential evapotranspiration (PET), which is one of factors that have great influence on the drought condition, is estimated by a set of models. The set consists of four temperature-decisive models (Hamon, Hargreaves-Samani, Oudin, Thornthwaite), two radiation-decisive models (Energy-Only and Priestley-Taylor) and two synthesis models (Penman and Penman-Monteith). Subsequently, Standardized Precipitation Evapotranspiration Index (SPEI) is applied to assess the spatial and temporal distributions of droughts for the arid, the semi-arid and semi-humid, as well as the humid climatic zones in China for 1961–2017. The results show that: (1) Taking China as a whole, annual PET estimated by Penman-Monteith (P-M) formula is about 1000 mm, which is in the middle of the range of results from eight algorithms. Annual PET values estimated by radiation-decisive methods are relatively close to those obtained by P-M, while temperature-decisive methods yield lower values and the largest disparity among the different PET models can be up to 2.5 fold. (2) Comparing to drought records, both P-M and Thornthwaite are suitable for assessing drought condition on the national scale, but the drought assessed by the Thornthwaite method is worse than that by the P-M method in 1990–2017 when warming is faster than before in the arid region. (3) Increase first and then decrease of values of the SPEI index are detected in China for 1961–2017 by all PET estimations. Averaged over entire China, the wetting trend has changed to drying in the early 1990s by P-M-based SPEI. More meteorological stations are found to experience droughts after the early 1990s, particularly in the western humid region and the central semi-arid and semi-humid region. In contrast, a wetness tendency is found in the arid region. (4) Patterns of the drought frequency and duration by all PET algorithms maintain relatively good correspondence in the semi-arid and semi-humid region as well as the humid region. But in the arid region, drought frequency obtained by P-M is lower than for radiation-decisive and temperature-decisive models, while drought duration is longer.