On the Comparison between Calculated and Observed Evaporation Amounts
1965 Volume 21 Issue 1 Pages 23-27
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1965 Volume 21 Issue 1 Pages 23-27
In this paper, the amount of observed evaporation was compared with those calculated with two methods. The data on evaporation were quoted from the new climatic tables for Japan published recently by meteorological agency. The evaporation from a shallow water was calculated by PENMAN method (E0) and socalled gradient method (E). Other theoretical and empirical formulas were not very suitable for our purpose, because of the need of detailed climatic data and of emirical constants that vary with places. It is reasonable to expect that the two methods give the same results since the both are on the basis of the principle of heat balance.
In the calculation of evaporation by PENMAN method, the data on incoming short wave radiation with Robitzsch actiongraph were used and the value of albedo of water surface was taken from Budyko's (1959) textbook. The following equation was used to caluclate the evaporation from water surface by the gradient method.
E=k{e(θ∝)-ed},
where k is the evaporation coefficient, e (θ∝) the saturation water vapor pressure (mb), ed the water vapor pressure at the shelter height (1.5m) and θ∝ the terminal water temperature (the same to daily mean of water temperature), The evaporation coefficient was obtained from the data on the heat transfer coefficient reported by several researchers and found to be 0.42mm/day. mb. The terminal water temperature was calculated by the heat balance method on the basis of climatic data.
The results so obtained are presented in Table 1. This table shows the fair agreement in amount between the evaporations calculated by the two methods except of the months with wind velocity higher than 3 or 4m/sec. The discrepancy between them in the period of wind velocity higher than 3 or 4m/sec seems to be due to the assumption of a constant value in the evaporation coefficient. As can be seen in Fig. 1, the ratio (E/Rn) and ratio (E/obs) show the remarkable annual variation with higher values in winter and relatively constant values in warm season. The value of ratio (E/Rn) in. warm season was found to be in the range 0.8 to 0.9 and it was somewhat larger than those obtained by other researchers.
In order to obtain a more simple equation than the two methods used here, the data on evaporation were treated and obtaind the following empirical relation.
E=Rn+1/0.06D-5.4, mm/day
where Rn is the net radiation equivalent (mm/day) and D the saturation deficit (mb). This relation is very convenient for the operational use.
The significant discrepancy in amount between pan evaporation and evaporation estimated by heat balance method seems to indicate that pan evaporation is not very reliable in scheduling the use of water resource,
