Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/7/2004
Publication Date: 5/13/2005
Citation: Kozak, J.A., Ahuja, L.R., Ma, L., Green, T.R. 2005. Scaling and estimation of evaporation and transpiration of water across soil textures. Vadose Zone Journal. 4:418-427. 2005. Interpretive Summary: The amount of water leaving the top soil through drying (evaporation) and the amount of water leaving the soil via plant uptake (transpiration) were examined with respect to eleven soil types, i.e. clay to sand. It was found that there is a strong relationship between evaporation and soil type drainage index (l) and transpiration and l. Because of this strong relationship, a mathematical model was developed that could estimate evaporation and transpiration based on soil type and potential rates. This study will help in understanding how the amount of soil water changes across a unit area due to the magnitudes of the above processes in order to develop agricultural site management. This model will also help in crudely estimating the evaporation and transpiration of a large watershed based on actual measurements of evaporation and transpiration of a small unit area, thereby limiting the number of field measurements.
Technical Abstract: Recently we showed all parameters in the Brooks-Corey equations of soil hydraulic properties are strongly correlated with pore-size distribution index (l), and l relates and can scale cumulative infiltration and soil water contents during redistribution across dissimilar textural classes under different rainfall and initial conditions. The objectives of this work were to explore if relationships exist between evaporation (E) and transpiration (T) and l across different soil types and can be used to scale E and T among these soils. The Root Zone Water Quality Model generated evaporation under four potential rates and transpiration under one potential rate with a goose grass in eleven soil textural classes under near-saturated initial conditions. Stage I cumulative evaporation or transpiration that occurs when the soil is sufficiently wet to meet the potential rates, had a quadratic relationship with l. However, both Stage II cumulative evaporation and transpiration were cubic functions of l with time-dependent coefficients. It is shown that these relationships can be used to estimate both Stage I and II cumulative evaporation and transpiration across unknown soils where data for one dominant reference soil type is known. The methods developed for estimating cumulative evaporation were applied and compared to experimental results of three initially saturated soils under constant evaporation with fairly good results. These results for simple cases of homogeneous soils should be useful in quantifying spatial variability of soil water evaporation and transpiration in the field under similar conditions, and could form the basis for further research of more complex conditions.