I would like to examine seasonal actual evapotranspiration as a function of potential evapotranspiration and soil moisture. I have some water balance information: precipitation, streamflow, spring flow, and volumetric water content but not groundwater recharge. I also have sufficient meteorological information to estimate potential ET. This is in a heterogeneous landscape and evapotranspiration is water-limited throughout most of the growing season. I have information regarding the bulk density, field capacity, and texture of some soils in the area. Are there any established methods that examine this relationship? Any suggestion regarding existing research into this topic, particularly for arid and semi-arid landscapes, is appreciated. Thank you!
The bulk density, field capacity and texture will only assist in determination of Readily Available Water. Potential Evapotranspiration is found using FAO-Penman-Monteith equation using the climatic factors. Actual evapotranspiration now will depend on the growth stages of crops.
Dear
In addition to available water in the root zone, actual evapotranspiration depends also on crop type and its phenological stage.
To better understand these issues, have a look at this detailed report :
https://www.researchgate.net/publication/235704197_Crop_evapotranspiration-Guidelines_for_computing_crop_water_requirements-FAO_Irrigation_and_drainage_paper_56
The answer to your question is in
Jensen M E, Wright J L, Pratt B J (1971). Estimating soil moisture depletion from climate, crop and soils data. Trans ASAE 14, 954- 959
I developed a conceptual water balance model and showed that it performed as well as any finite element solution of the Richards equation. A smart Chinese post-grad changed a Lotus 123 version of the software to SQL for web use. I'm no longer in China but hope that the software is being used there.
You may be able to get access to a simple water balance model. My SQL software
I should have said you need to be prepared to challenge orthodoxy. There are no basic physical equations to exactly describe the phenomena of evapotranspiration or water & vapour transport in soil.
Both the Penman and Penman Monteith equations are equally valid empirical estimates describing out best experimental determinations of soil water change.
The last addition was incomplete. The Richards equation does not fully describe the measured phenomena so conceptual representations are equally valid in my experience. The real physics must include air and vapour flow.
I should also have given you my last "accepted" paper :-)
The piecewise linear relationship of Laio et al. (2001) is used to estimate the rate of actual evapotranspiration from the available soil moisture in the root zone and can be related to the potential evapotranspiration (PET) in various semiarid landscapes. Here I have given the link of my recent work, where I have used the BGM model to utilize the soil moisture conditions for estimation of the actual evapotranspiration at various latitudes that fall in semiarid landscapes:
Article The Role of Landscape Morphology on Soil Moisture Variabilit...
Laio, F., Porporato, A., Ridolfi, L., & Rodriguez-Iturbe, I. (2001). Plants in water-controlled ecosystems: active role in hydrologic processes and response to water stress: II. Probabilistic soil moisture dynamics. Advances in water resources, 24(7), 707-723.
I hope it helps
Cheers,
Ankur
I should have said that one of the modelling assumptions
was that no surface run-off occurred, in the context of irrigated
annual crops.
If it's helpful I validated scheduling software in China, initially written for commercial use in Scotland.
Thank you very much for all of the information, including the references. I appreciate it!
You're welcome Nicole. It's interesting that Dr Rene Brookes at OSU has independently developed a similar conceptual water balance model for forestry based on oxygen isotopes - so-called "meteoric water". best martin
In response to Ankur's comments, I haven't researched regional soil water variability where runoff is significant factor. However, the appropriate tool for model validation would seem to be the cosmic ray neutron sensor.
best martin parkes
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