Spatially distributing monthly reference evapotranspiration and pan evaporation considering topographic influences

Many hydrological models engage spatially distributed measures of 'potential evapotranspiration' (ET_pot). The reliability and utility of the physically based Penman-Monteith approach to generate ET_pot has been recently advocated. Assuming land-surface conditions, spatial surfaces of reference evapotranspiration (ET₀) can be generated taking into account the topographic influence of forcing meteorological variables. This was performed in this paper by spatially interpolating maximum (T_max) and minimum (T_min) air temperatures, wind speed (u) and vapor pressure (e_a), using a spline model with a linear sub-model dependency on elevation, and modelling the radiation environment, taking topography (i.e., elevation, slope and aspect) into account, prior to calculating ET₀ at each grid-cell. In accordance with previous research, resultant lapse rates showed a strong seasonal pattern; values were steeper in summer than winter and those for T_max were steeper than for T_min. Monthly mean T_max lapse rates varied from -3.01 °C km^-1 in winter to -7.69 °C km^-1 in summer, with T_min lapse rates ranging from -2.79 °C km^-1 in winter, to -6.64 °C km^-1 in summer. Monthly climatologies of the near-surface elevation-dependence (NSED) for u and e_a also showed strong seasonal values. NSED of u varied from 2.01 ms^-1 km^-1 in winter reducing to 0.75 ms^-1 km^-1 in summer. The NSED for e_a ranged from -0.08 kPa km^-1 in winter to -0.64 kPa km^-1 in summer. For a 252-month sequence from 1980 through 2000, spatial surfaces of ET₀ with a 100 m resolution for the 113,000 km² study site located in the Loess Plateau, China were generated using an 'interpolate-then-calculate' approach. Resultant ET₀ values varied from about 20 mm month^-1 in winter to over 150 mm month^-1 in summer. In order to assess the reliability of these ET₀ surfaces, pan evaporation (E_pan) was also spatially interpolated and from these a set of pan coefficient (K_pan - a unitless ratio defined as ET₀/E_pan) surfaces were calculated. Spatio-temporally averaged K_pan values for the study site varied from 0.44 in April to 0.65 in late summer. K_pan values were in agreement with another study using a Chinese 20 cm diameter micro-pan, and, as expected, were lower than other values documented using a Class A pan. The influence of topography, especially aspect, was seen on the resultant ET₀ and K_pan, but not E_pan, surfaces. Sensitivity analysis showed that results were particularly stable in the hydrologically active portion of the year extending from March to October, inclusive. This study demonstrated that high spatial resolution monthly surfaces of ET₀ can be spatially modelled while taking into account the influence of topography on the forcing variables.