Daytime partition of irradiation and the evaporation chilling of the ground

Abstract

The present study concerns daytime conduction and convection at the ground; how added soil moisture magnifies the heat‐flow cycle; and how latent heat of vaporization offsets the diurnal temperature cycles. Measured diurnal cycles of net radiation, conduction, and convection for dry ground are formalized to show how the amplitudes and phase lag of half‐cycle temperature and conduction waves indicate the partition of energy at the ground surface. A series of four test plots of five acres each was studied during the spring frosts in a pear orchard near Lakeport. Two plots were disk‐cultivated, and one of these was immediately flood‐irrigated lightly. The observed diurnal magnitudes of heat flow were more than twice as great in the watered plot as in the unirrigated. The striking results are not the effect of disk cultivation but rather that of the chilling caused by application of water. The watered plot took more than ten days (in April) to recover from the evaporation chill (but then warmed up more than the other three plots and two weeks after flooding was definitely less liable to frost than the unwatered plots). These investigations have the considerable advantage of direct measurement of heat‐conduction rates into and out of the ground. Much remains to be done, however, to establish empirical coefficients to be used with simplified formulas. The measurement of evaporation is an important future objective, but knowledge of total daily water loss from the soil can be used in the formulas to get a fair understanding of the diurnal thermal system.