Soil heat flux in the Penman-Monteith evapotranspiration equation

Some reliable evapotranspiration-estimating methods, like Penman-Monteith's (Monteith, 1973), require knowledge of the available energy expressed by the difference between net radiation (R-N) and soil heat flux (G). G is considered as either zero or portion of R-N, changing with the crop development and, thus, with the leaf area index (L). Establishment of a relationship between G/R-N and L is attempted here both for day and night time during the development of a crop, under varying soil moisture regimes. A reliable exponential relation between the two parameters, applicable for crops with varying geometry and architecture of canopy, is proposed for day time. With L approaching zero, G/R-N tends to the value 0.43, whereas for large L, the ratio approaches its limit value 0.1. At night, G/R-N and L are related linearly for L>2, but for smaller values of L, G approaches R-N.