Improvements to the accuracy of modelled soil water content from the Second Generation Prairie Agrometeorological Model

Abstract

Gervais, M., Bullock, P., Mkhabela, M., Finlay, G. and Raddatz, R. 2010. Improvements to the accuracy of modelled soil water content from the Second Generation Prairie Agrometeorological Model. Can. J. Soil Sci. 90: 527Á542. The direct measurement of soil water on a regional scale is often not practical due to large instrumental and labour requirements. Alternatively, soil water estimates can be derived using models. The Second Generation Prairie Agrometeorological Model (PAMII) models soil water, crop development and evapotranspiration (ET) in order to derive an estimate of crop water use. The objective of this study was to validate, and if necessary modify, the soil water component of PAMII using weather and soil water data collected from several spring wheat trials in Saskatchewan and Manitoba during the 2003 though 2006 growing seasons. Comparison of modelled and measured soil water values yielded a RMSE of 62 mm. For most site-years, PAMII overestimated soil water during the second half of the growing season, which was caused by an increase in modelled canopy resistance (r c ) before the crop experienced water stress. The r c function was thus modified so that r c would not increase until the soil water content was B0.5 of plant available water. Overall this modification reduced the RMSE from 62 to 56 mm. In addition, modelled soil water was underestimated during periods that experienced consecutive days of precipitation. This was because the model stopped infiltration when the top-zone reached saturation. When modified to allow infiltration to continue independent of the top-zone's water content, the RMSE was further reduced to 53 mm. Overall, both modifications reduced the RMSE of modelled soil water by 9 mm, and this reduction was highly significant (PB0.01).Key words: Prairie Agrometeorological Model (PAMII), soil water modelling, evapotranspiration, Canadian prairies Gervais, M., Bullock, P., Mkhabela, M., Finlay, G. et Raddatz, R. 2010. Ame´lioration de la pre´cision de la teneur en eau du sol e´tablie graˆce au mode`le agrome´te´orologique des Prairies de 2 e generation. Can. J. Soil Sci. 90: 527Á542. Mesurer directement la teneur en eau du sol a`l'e´chelle re´gionale est rarement pratique en raison de l'importance du mate´riel et du travail que cela requiert. Paralle`lement, on peut de´river une estimation de ce parame`tre graˆce a`divers mode`les. Le Prairie Agrometeorological Model de 2 e ge´ne´ration (PAMII) mode´lise la teneur en eau du sol, la croissance des cultures et l'e´vapotranspiration (ET) pour estimer la quantite´d'eau utilise´e par les plantes. La pre´sente e´tude devait valider et au besoin modifier la composante du PAMII relative a`la teneur en eau du sol a`partir des donne´es me´te´orologiques et des releve´s de la teneur en eau recueillis a`l'occasion de plusieurs essais de culture du ble´de printemps effectue´s en Saskatchewan et au Manitoba, durant la pe´riode ve´ge´tative, entre 2003 et 2006. En comparant la teneur en eau du sol mode´lise´e a`celle e´tablie par mesure directe, o...

Comparison of standard and actual crop evapotranspiration estimates derived from different evapotranspiration methods on the Canadian Prairies

“…With the exception of Regina in 2006, PAMII 1+2 had higher cumulative ET a estimates compared to the FAO56 P‐M method in 2003 and 2004, but lower estimates in 2005 and 2006 (Table I). The lower ET a estimates by PAMII 1+2 in 2005 and 2006 are likely a result of a greater proportion of precipitation being allocated as deep drainage as a result of soil water content exceeding field capacity and therefore less plant available water for ET (Gervais, 2008; Gervais et al , 2010). As previously mentioned, both PAMII 1+2 and FAO56 P‐M use similar approaches in regulating ET a as a function of plant available water.…”

“…L A is the crop's fractional leaf area which has a minimum value of 0·3 when used in Equation (). Canopy resistance for PAMII 1+2 starts to increase once soil moisture has been depleted beyond 50% of plant available water (Gervais, 2008; Gervais et al , 2010).…”

“…The models normally utilize a measured soil water content at the beginning of the growing season and then estimate daily soil water content for the remainder of the growing season to derive crop ET a . Since there is error in the daily soil water estimates from the models (Gervais et al , 2010), the value of modelled ET a in a given period could be adversely affected by errors in soil moisture estimation that resulted prior to the period. Therefore, the water content for both models was reset at the beginning of each period of measurement (i.e.…”

“…daily minimum and maximum surface air temperature and daily rainfall available at most weather stations as well as the state of the atmospheric boundary layer, which can be interpolated from the regional Global Environmental Multiscale (GEM) model gridded output run by the Meteorological Service of Canada (Raddatz, 1993). PAMII 1+2 (Gervais et al , 2010) is a modified version of the original second‐generation Prairie Agrometeorological Model (PAMII) (Raddatz, 1993), which itself is a modified version of the first‐generation Prairie Agrometeorological Model (PAMI) (Raddatz 1989). Modifications to PAMII (to come up with PAMII 1+2 ) included changes in the way canopy resistance and water infiltration are modelled (Gervais, 2008; Gervais et al , 2010).…”

“…PAMII 1+2 (Gervais et al , 2010) is a modified version of the original second‐generation Prairie Agrometeorological Model (PAMII) (Raddatz, 1993), which itself is a modified version of the first‐generation Prairie Agrometeorological Model (PAMI) (Raddatz 1989). Modifications to PAMII (to come up with PAMII 1+2 ) included changes in the way canopy resistance and water infiltration are modelled (Gervais, 2008; Gervais et al , 2010). These modifications improved the simulation of the soil water balance, which in turn increases our confidence in the accuracy of the modelled ET c estimates.…”