Characteristics of Albedo over a Potato Field

Abstract

Albedo is an important factor which is related to the radiant and thermal environments. There are many works on the observation of albedo on various crop surfaces, and some literatures indicate that the albedo is affected by soil moisture, leaf area and so on. However, these results have not been determined quantitatively. It is the purpose of this paper to present the relationships among albedo and soil moisture, leaf area and weather condition on a potato field quantitatively.
The albedo on the potato field was related with the ratio of green canopy area (i.e. percentage of the area covered with green canopy to the total field area), the weather condition and the wetness of soil surface. In this paper, the dry spell days was used to describe the moisture condition on the soil surface, and the weather condition was represented by the extraterrestrial radiation ratio (i.e, ratio of solar radiation on the field to extraterrestrial solar radiation). The dry spell days is the number of days after the rainy day which has more than 15mm of rainfall. In case of less than 15mm of rainfall, the dry spell days was decided by using the relation between the wetness of soil surface and the dry spell days which was obtained in case of more than 15mm of rainfall in advance.
Calculation of albedo from daily totals of imcoming and reflected solar radiation indicated a range of 9% over wet soil to 12% over dry soil, and up to 21% over crop surface. At early growing season, the albedo was affected considerably by both the ratio of green canopy area and the dry spell days. However, the albedo at latter growing season which was more than 60 percent of the ratio of green canopy area, was not almost affected by the dry spell days. The effect of extraterrestrial radiation ratio on albedo was not significant all over the experimental period.
The equations to estimate albedo were drived by using both statistical method and another method. These equations need the values of ratio of green canopy area, dry spell days and extraterrestrial radiation ratio. The statistical equation, Eq. (2), brought about reasonable results on the estimation of albedo, but the behavior of expected albedo from Eq. (2) did not correspond with the real behavior when the ratio of green canopy area became larger. On the contrary, another equation, Eq. (8), suggested that it will bring about close agreement between both estimated and measured albedos under the condition of all ratio of green canopy area.