I'm going to apply the FAO's equation for the wind climate erosivity of the Tibetan Plateau, but the potential evapotranspiration (PET) is sometimes less than precipitation (P) in this region. I'm wondering if the negative value calculated in this case has any particular meaning. Obviously, it will affect the aggregation of monthly data to annual data.
It’s been years since I actively did some water balance study and calculations in Missouri. I have not used the specific equations mentioned and used Thornthwaite Mather approach in estimating PET. Pertaining to PET is less than Precipitation, I don’t see a problem, especially if there is streamflow. And there are delayed response issues associated with snow and ice, which could occur as precipitation in one month, and then not melt or evaporate/transpire or be released from groundwater sources until some later month. Deep seepage and changes in soil moisture storage are also difficult to assess using monthly data. You may still have some months where PET > P. Any streamflow during those months is primarily from groundwater and declines in soil moisture storage unless rainfall intensity is great enough to generate direct runoff.
You might be be interested in looking up one of Luna Leopold papers in SW USA that discussed the periods of drought about every 20 years (where PET likely >> than P) and comparing to wet periods. Dry periods had much higher erosion than wet periods. The difference was dry periods did of course have less rain to drive erosion, but the plant cover was so much less that erosion was greater. You may also find in other studies that hot wildfires or drought periods in some soils create hydrophobicity in the soils, which can increase runoff and erosion.
Dear William F. Hansen , Thank you for your answer.
I think your explanation makes a lot of sense, and it is common for PET to be less than P. However, this has caused me confusion in applying this equation. I chose this particular equation to calculate the index of land sensitivity to wind erosion, and this index can help me to continue analyzing other ecological issues (e.g., exploring the community characteristics of wind erosion sensitive areas), i.e., I may not need quantitative erosion values. Since I am not an expert in soil erosion, I wanted to find a suitable equation to simplify the effect of climate on wind erosion. If other conditions are consistent, the land will be sensitive to wind erosion in drier times and insensitive to wind erosion in wetter times, which is what this equation is trying to convey. I am not sure if this equation is applicable to my study area, and am also looking for a better solution.
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