Graduate Thesis Or Dissertation
 

Analysis of a hydrologic balance model and Penman-Monteith evapotranspiration estimating methods

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  • The major objectives of this study were an evaluation of the Meteorological Office Rainfall and Evaporation Calculation System (MORECS) evapotranspiration (ET) model using the Penman-Monteith equation and comparison of the MORECS and measured surface resistance. Meteorological, soil and plant parameters were measured in an alfalfa field over two growing seasons. Soil water content was measured daily using the neutron probe. Stomatal resistance was measured with a diffusion porometer two days per week and leaf area was measured with a portable areameter. The ET obtained using the MORECS model was compared with that calculated using a hydrologic balance model (Cuenca, 1988) and the Wright- Penman equation (Wright, 1982). The hydrologic balance model applied on a daily basis produced large variations in estimated ET both among individual access tubes and from day-to-day. Drainage compensation, dew compensation and moving average smoothing procedures were tested to reduce this high variation. Completely satisfactory results were not obtained with any of these smoothing procedures. An error analysis was performed to identify sources of variation in depth of stored soil moisture. The variation was associated with inaccuracy in the neutron probe calibration equation and variance in probe readings at a given time and location. The standard error of depth of stored water is reduced with an increase in the number of neutron probe measurements per depth layer. Four readings per monitored depth layer would reduce this standard error by approximately fifty percent. The total variance in depth of stored water over the soil profile is also reduced by decreasing the depth layer thickness. The MORECS model underestimated surface resistance early in the growing season. The difference between MORECS and measured surface resistance was negligible at full cover. Allen's (1986) empirical equation overestimated the surface resistance for the total growing season. An average minimum surface resistance of 18 s/m was found for alfalfa growing in Corvallis. A difference of approximately 10 percent in the cumulative ET over a period of two months was found using the MORECS minimum surface resistance of 40 s/m compared to a measured value of 17.6 s/m. The most accurate ET estimates therefore require a locally determined surface resistance.
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