Figure 1: Hydrologic cycle in relative flux units (100 units equals marine evaporation). Figures have been rounded up. (adapted from Chow 1964)

Figure 3: The Craig-Gordon isotopic evaporation model. I and B signify the surface interphase zone and the atmosphere boundary layer respectively; x = (1 hN) and y = (aV/LRL hNRA) where hN is the relative humidity normalised to the saturated vapour pressure at the temperature and salinity conditions of the water surface and dA its isotopic composition; d`A is the isotopic composition of the air moisture at the boundary layer of the diffusive sub-layer and h` is the corresponding relative humidity.

Figure 9: The precipitation input

Figure 18: The evolution of the isotopic composition of a number of desert showers (Sde Boker, Negev desert) and the isotopic composition of the resulting local run-off and regional flood. It is evident that the flood selects the most depleted part of the rainfall.

Figure 21a: Schemes of different combinations of evaporation and transpiration areas on the isotopic composition of the residual run-off. FP the rain flux; z the lake runoff fraction ie the fraction of inflow to the lake which runs off as surface flow; y the total run-off fraction of the basin; x the fraction of precipitation on the basin which is re-evaporated from the lake; and t the fraction of precipitation which is recycled by transpiration (from Gat and Matsui, 1991.)

Figure 24: The isotopic composition (dL,SS ) of lakes with varying through flow rates and the equivalent value of the evaporate (dE). x = Fin/E where x = 1 obviously refers to the terminal lake, where all the inflow is lost to evaporation under steady state conditions.

Figure 27a: The continental scale isotope balance:- The bird-eyes view: Under a simple and ideal Rayleigh-rainout scenario the atmospheric moisture is depleted in the heavy isotope content commensurate with the loss of moisture by precipitation during a continental passage, following more-or-less a Meteoric Water Line with a constant d-excess. Due to the eco-hydrological interactions at the land surface part of the incoming precipitation is re-evaporated, either directly from open water bodies and canopy intercepted precipitation pools, from the topsoil layer or through the intermediary of plants as transpiration. The latter, i.e. the transpiration flux, returns the water essentially unfractionated to the atmosphere with the result that there is an apparent reduction in the degree of the Rayleigh rainout effect, which can be measured by comparison of the isotopic buildup in the atmospheric waters compared to the expected buildup based on the precipitation amounts. Evaporation from open water surfaces returns a fractionated flux, where the residual surface waters are enriched and the evaporated moisture depleted in the heavy isotopes of Hydrogen and Oxygen, following an Evaporation Line rather than the Meteoric Water Line. Under such circumstances the degree of recycling of the moisture due to this mechanism can be quantified by recording the change in the d-excess parameter in the downwind atmospheric waters, In the more arid environment with its endorheic runoff regime, more and more of the runoff is lost by evaporation as the scale of the basin increases [6], often terminating in highly saline lakes or sabkhas. In contrast to the situation described above, the evaporative signature of a decreasing d-excess is then accentuated in the runoff with increasing scale of the system whereas the effect on the atmospheric moisture diminishes as a higher and higher fraction of the surface water is evaporated. The distinction between the evaporation and transpiration fluxes based on the water and isotope balances, which is a very valuable diagnostic tool under humid and semi-arid conditions, cannot then be indiscriminately applied in the more arid environment.

Figure 27b: The Continental Scale Isotope Balance

The view from below

The surface and sub-surface runoff on a basin or continental scale carries the integrated precipitation isotope signal, alas at times with a considerable time delay. As the geographic scale increases the effect of the evaporative signature of the watershed processes is diluted in the continual runoff by the precipitation in excess of the infiltration capacity of the soils. In the more arid environment with its endorheic runoff regime, more and more of the runoff is lost by evaporation as the scale of the basin increases, often terminating in highly saline lakes. In contrast to the situation in the atmosphere, the evaporative signature of a decreasing d-excess is accentuated in the runoff with increasing scale of the system.