Reference evapotranspiration with radiation-based and temperature-based method - impact on hydrological drought using WATCH Forcing Data

Abstract

In this report, two di erent methods to calculate reference evapotranspiration are applied and compared. This has been done in response to inconsistencies among the daily variables in the WATCH Forcing Data (WFD). One method is so-called radiation based (ET0rad), and is the well known Penman-Monteith equation with, among others, incoming short wave radiation as a variable. The radiation-based method used the variables Tair, Tmax and Tmin, Wind, SWdown, LWnet, Qair and PSurf from the WFD. The other method is so-called temperature-based (ET0temp), the radiation term is replaced by an approximation of the radiation based on minimum and maximum air temperature. The temperature-based method only used the variables Tair,Tmax, Tmin and Wind from the WFD. After calculating reference evapotranspiration ET0 with both methods, it is fed into a conceptual hydrological model that combines a soil water balance and a simple lumped groundwater model. The model generates a daily water balance, e.g. potential evapotranspiration (ETP ), actual evapotranspiration(ETA), soil moisture storage, groundwater recharge and groundwater discharge. Eventually, the simulated groundwater discharge is used to de ne periods of drought with a Variable Threshold (VT) method. The VT in this study used Q80, implying that a period is designated as drought as soon as the groundwater discharge is in the lowest 20% of all simulated discharges for that particular month. Drought periods and characteristics, like length and de cit, are identi ed. The model using the two di erent ET0 series, is applied to 1495 cells (land grids) that well represent the ve K oppen-Geiger major climates across the world. First, a comparison is made between the ET0rad and ET0temp. There are clearly di erences between the two methods, and generally the radiation-based method leads to higher ET0 than the temperature-based method. There are some exceptions within the di erent major climates, and also the northern and the southern hemisphere behave di erently in this perspective. The di erences between ETArad and ETAtemp are signi cantly smaller than for the reference evapotranspiration. In general the radiation-based method still leads to higher ETA than the temperature-based method, but the di erence is smaller and in many cases the di erence even decreases to zero (26% of the cells globally). Finally, the di erent drought characteristics are compared. There are remarkable di erences in drought characteristics, especially in de cit volume and intensity, but in general the di erences are within ranges found in other literature that describes the impact of hydrological models or datasets with di erent diurnal forces on hydrological drought. Climate D and E, the snow-a ected climates, show the largest di erences.