A terrestrial focus

Climate of the second kind and the terrestrial branch of the hydrologic cycle embrace the suite of natural processes at or near the land surface that account for the conversion of precipitation into streamflow. The primary variables involved with these processes are fluxes represented by precipitation, evaporation and evapotranspiration, and runoff. These fluxes of energy and mass related to land surface processes are dominantly oriented upward or downward relative to the land surface, and they are aided by energy and mass storages and sinks occurring at or near the land surface. Wind and soil moisture are additional variables that augment or dampen the rate of energy or mass exchange between the land surface and the atmosphere. Wind is a horizontal flow that is a fundamental factor influencing evaporation and evapotranspiration. Soils have non-linear flow properties that transform sudden changes in land surface conditions into gradual changes in subsurface water movement. In addition, soil moisture storage provides water for plant transpiration during periods between precipitation events.

Efforts to generalize the characteristics of the energy and mass fluxes defining climate of the second kind and the terrestrial branch of the hydrologic cycle are complicated by the complexity of the Earth's land surface. The land surface supports an array of heterogeneities and discontinuities related to forcing inputs, state conditions, and land surface properties that influence moisture processing. Heterogeneity of the land surface is obvious in soils, vegetation, and topographic differences at various spatial scales (Becker, 1995).