H33B-1259
PenPan: A general tool for the attribution of changing pan evaporation
Evaporative demand is routinely measured using the evaporation of water from standardised pans. The most common are Class A pans: they are metal dishes 4 feet in diameter and 10 inches deep sitting on a wooden platform. Because of the practical importance in agriculture and engineering, there is a wealth of pan evaporation data. Analysis has shown that in most places, pan evaporation has declined over the last 30 to 50 years despite the well documented increases in near–surface air temperature. It is important to understand why this has occurred. Using generic principles of mass and energy balance, we developed the PenPan model of evaporation from a class A pan for attribution purposes. In this talk we briefly describe the PenPan model and then demonstrate its application at 41 sites, broadly scattered across Australia, for the period 1975-2004. We found that the decline in Australian pan evaporation was mostly due to declining wind speed with some regional contributions from declining radiation. In general, the observed changes in vapour pressure deficit and air temperature were too small to have an appreciable impact on pan evaporation rates. By a comparative analysis, we show that the general global trends of declining evaporative demand are most due to various combination of declining radiation and/or wind.
H33B-1260
A rational function approach for estimating land surface evapotranspiration based on the complementary hypothesis
Evapotranspiration, which occurs in the boundary layer between the land surface and the bottom atmospheric layer, plays an important role in both water balance and energy balance. Models based on the Penman hypothesis (1948) and the Budyko hypothesis (1974) estimate actual evapotranspiration from a land surface process prospective, while models based on the complementary hypothesis (Bouchet, 1963) do this from the atmospheric perspective. Penman-based models require detailed data on soil moisture or stomatal resistance (Crago and Crowley, 2006); Budyko models, e.g. Fu's equation (1981), estimate the mean annual evapotranspiration only; while models based on the complementary hypothesis, including advection aridity model (AA for short) (Brutsaert and Stricker, 1979) and the Granger model (1989, 1991, 1996) estimate actual evapotranspiration at various time scales using climate data only. The AA and Granger models use different definitions for wet environment evaporation and potential evaporation and their comparative study are conducted by several researchers (Xu and Singh, 2005; Liu et al., 2006; Crago and Crowley, 2006). In this paper we explore the uniformity of the two complementary models by dimensional analysis. A new index (the proportion of the radiation term in Penman equation, termed the air humidity index) is proposed as a measure of the wetness of the evaporating surface via the wetness of over-passing air, and a general functional form for actual evaporation is developed in which the evaporation ratio is expressed as a function of the air humidity index. The similarity and differences between the AA and Granger models are interpreted via this rational function approach, and a new power function method is proposed. The theoretical analysis is confirmed by observational data under various climate conditions.
H33B-1261
Climate Model Simulations of Point Potential Evaporation
This paper examines whether the observed point potential evaporation (Ep) climatology and trends are reproduced in an offline simulation based on Global Climate Model (GCM) output. Ep is a measure of potential evaporation from an area which is so small that the effects of the evaporation on the overpassing air would be negligible. For example, an evaporation pan measures the evaporation at a point, and is too small to modify the temperature and humidity of the passing air when it becomes well mixed. This work is essential to reconcile the difference between observations and model simulations, namely there have been negative trends reported in the recent observed pan evaporation in some regions (e.g. Australia and New Zealand) while there have been positive trends projected in climate model simulated evaporation for the future (e.g. in Australia). In this study, the simulated Ep was calculated using the Complementary Relationship Areal Evaporation (CRAE) model of Morton (1983). The required inputs - air temperature, humidity measures (vapour pressure, dew point temperature, relative humidity) and downward solar radiation at the surface - are obtained from a number of GCMs. The examination of model performance in simulating observed Ep climatology is conducted on a global basis, while that of model performance in simulating observed Ep trends is conducted over the Australian region.
H33B-1262
Trends in Pan Evaporation and Application of the Complimentary Relationship of Evaporation in the Great Basin, USA
Evaluating available water resources in the Great Basin is an increasing topic of discussion by several local, State, and Federal government agencies due to proposals for water importation into areas of rapid population growth. In estimating a water budget for a particular flow system in the Great Basin, groundwater evapotranspiration (ET) from phreatophyte vegetation is often the most important as it is the only water budget component that can be estimated with some certainty, opposed to mountain block recharge and interbasin subsurface flow. Bouchet's complimentary relationship of evaporation has drawn noteworthy attention in recent years, not only because of its use in explaining observed changes in the hydrologic cycle, but also for its use in predicting ET using readily available meteorological data. The objective of this presentation is twofold: 1) to discuss pan evaporation trends in the Great Basin, and 2) discuss results from the application of the complimentary relationship for predicting evapotranspiration from phreatophyte shrubs when compared to measured ET at several U.S. Geological Survey eddy correlation and Bowen ratio sites. Initial findings indicate a decrease of pan evaporation in Death Valley, CA, the lowest, hottest, and driest location in North America, an opposite trend from analysis of several other sites in the Great Basin that indicate an increase of pan evaporation. It is believed that these opposing trends are likely the result of local water availability surrounding the pan, and or data quality. The application of a symmetric complimentary relationship indicate satisfactory results when compared to measured evapotranspiration, however, during the hottest months of the year the predicted ET over estimates the measured. Further investigation of micrometeorological data suggests that an asymmetric complimentary relationship between potential and actual ET exists, and when taken into account, improves the predictions markedly.