Evaporation from bare soil following rainfall
Date
1999
Authors
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ORCID
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Degree Level
Masters
Abstract
The semi-arid environment experiences a potential for evaporation that
exceeds the supply of precipitation. In addition a significant portion of
precipitation falls in small daily amounts that might not be sufficient to counter
evaporative demands of the following day. The objective of this project was to
establish the relationship between daily rainfall amounts and that portion of the
rainfall lost to evaporation in the subsequent 24-hour period.
Evaporation is the link between the hydrological cycle and the surface
energy budget, therefore the evaporation evaluation techniques were based on
both the soil water balance and the energy balance. The energy balance methods
included the Bowen ratio, and the Penman-Monteith and G-D equations. The soil
water balance was evaluated with precipitation measurements as well as replicates
of two different types of microlysimeters that allowed evaluation of the drainage
component. The dataset consisted of daily soil moisture changes from
microlysimeters, soil moisture measurements, and daily rainfall; additional
measurements included net radiation, ground heat flux, air temperatures, relative
humidities, and windspeed. This dataset spanned 36 days in August and early
September 1994.
The microlysimeters that permitted drainage were deemed to be more
representative of field conditions, as they allowed the drainage during rainfall to
be quantified, but the drainage component after rainfall was difficult to isolate.
The replicated microlysimeters produced a spatially averaged estimate of
evaporation.
Microlysimetric and energy balance methods gave evaporation estimates
of 43.1 to 64.4 mm during a time when the precipitation received was 59.9 mm.
The two methods deemed to most accurately reflect actual evaporative losses
were the G-D and cotton-capped (corrected for drainage) microlysimetric
methods which produced evaporative loss estimates of 43.1 and 47.1 mm,
respectively. Estimates of subsequent 24-hour evaporation by the two methods
indicate 20.1 and 22.2 mm, respectively, of the 59.9 mm of precipitation was lost
on the day following rainfall. The dataset was limited and no rigorous regression
techniques could be used to establish a quantifiable relationship between daily
rainfall amounts and subsequent 24-hour evaporative loss, but the portion of daily
rainfall that could be viewed as effective precipitation decreased with decreasing
daily depths of rainfall.
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Degree
Master of Science (M.Sc.)
Department
Agricultural and Bioresource Engineering
Program
Agricultural and Bioresource Engineering