Standard Pan Research Articles

A Comparison of Some Methods for Estimation of Potential Evaporation

In most of the important agricultural regions of the world, evaporation in the growing season exceeds the precipitation, and also often the sum of precipitation and plant available water in the root zone. This causes a need of supplemental irrigation for effective agricultural use. Information about the greatest possible water consumption in given periods, therefore, is important for the water policy of a given region. Several methods for estimation of the maximum (potential) evaporation have been suggested, and many methods have been compared earlier (W.M.O. 1966, Heldal 1969, Kristensen 1971, Iruthayaraj and Marachan 1978). In Denmark a screened sunk iron pan has been used as a standard since 1956 (Aslyng and Hansen 1960). This standard pan and other methods for estimating potential evaporation have been compared during a 15-year period. The results are reported and discussed.

Drought Resistance Characteristics of Blue Grama Seedlings1

AbstractBlue grama (Bouteloua gracilis (H.B.K.) Lag. ex Steud.) seedlings often expand leaf area to the maximum that can be supported by water from the seminal root and then succumb to an increase in transpirational stress. The objective of this study was to define drought resistance of seedlings in relation to stages of shoot and root development. The changes in drought resistance that occur because of changes in seedling morphology have not been considered in previous quantifications of drought resistance. This information was essential for solving problems of seedling establishment on the shortgrass plains.Seedlings restricted to the seminal root, one adventitious root, or two adventitious roots were grown in moist soil in a controlled environment chamber, in the greenhouse, and in the field to further define seedling‐environment relationships. Relationships between drought of the shoot environment, maximum rate of water uptake, and maximum leaf area were investigated. Evaporation rate from a standard pan was used as a measurement of the drought of the shoot environment.Growth drought resistance, defined as the drought of the shoot environment that is just sufficient to halt expansion of seedling leaf area, was directly related to the critical evaporation/transpiration ratio for seedling growth and to the maximum rate of water uptake by the root; it was inversely related to seedling leaf area. In an environment with a constant evaporation rate, leaf area of seedlings had expanded to the limit of water supply when evaporation from a free water surface had exceeded transpiration rate six‐fold. Since growth drought resistance can be measured for individual seedlings, it will be useful in screening species and cultivars for seedlings with superior drought resistance, as well as for defining planting requirements.Survival drought resistance, defined as the drought of the shoot environment that is just sufficient to cause 50% death of seedlings, has only limited usefulness because of practical problems in its measurement. Large numbers of seedlings and a wide range of conditions are needed to accurately determine the relationship between drought and seedling death.

The heat balance of class A evaporation pan

The heat balance of a U.S. Weather Bureau class A evaporation pan located near Tucson, Arizona, was calculated for selected periods during the interval July 26–31, 1963. The method used required two pans, one of which was insulated on the side and bottom to prohibit heat transfer through these surfaces. For 10 of the 14 observation periods, heat transferred into the uninsulated pan via the side or bottom accounted for 29% of the total heat added to the pan. In the remaining periods, 6% of the heat leaving the pan was transferred through the side or bottom. Elimination of this heat transfer in the insulated pan resulted in an average daytime water temperature 2°C lower than that in the standard pan and a reduction of the daily evaporation rate by 28%. The data show the necessity of taking into account the heat transfer through the walls of the pan, especially when evaporation of natural water bodies is estimated from pan evaporation data. The heat balance method described offers one way in which the magnitude of this heat transfer can be determined. (Key words: Evaporation; lakes; meteorology; hydrology.)