In these closed basins, the runnoff returns to the atmosphere via evaporation that occurred in the lower and terminal part of these basins. The water balance is controlled by surface discharge into lakes or terminal lagoons (where water is lost by evaporation), groundwater discharge via springs (that eventually can reach terminal lagoons) and via diffuse evaporation in the unsaturated zone. The evaporation represents the average discharge that results of the water balance of the basin.

Therefore, it is extremely important to evaluate evaporation rates in order to evaluate the renewable water resources that can be used in these basins.

This article analyzes the main factors that play a role in evaporation and discusses the results of different approaches used to evaluate evaporation in the terminal part of closed basins in Northern Chile.

This work was done in 1990 and its results are currently applied in water balance studies in Northern Chile. The mining sector is the main user of water resources in the closed basins. If the spring discharge that supplies water to the lagoons is impacted, the water volume is replenished using groundwater via pumping wells.

Evaporation from saline waters
The natural evaporation affecting water in the terminal lagoons increases significantly the water salinity. The process of evaporation needs enough energy from solar radiation to transform the liquid into a gas phase that finally is lost to the atmosphere.

In general terms, the evaporation rates from a water surface (Ew) can be calculated using the following equation: Ew = k (Pw – Pa) , where k is the mass transfer coefficient from the water to the air, Pw is the vapor pressure of the water surface and Pa is partial water vapor pressure in the air. As the water salinity increases, the water pressure diminished and the evaporation decreases. Warren (1966) indicated that for the same meteorological conditions, the difference in evaporation rates between fresh water (Ew) and saline water (Eb) is proportional to the difference between the equilibrium temperature (Tb – Tw). Then, as the salinity increases, the magnitude of the temperature difference can increase which can reduce significantly the evaporation rates from the saline waters.

In order to take into account the effect of salinity in natural water, the present study used evaporation data obtained in saline water in experiments (Figure 1) carried out in the Salar de Atacama (II Region, altitude 2,300 masl). These data were used to correct evaporation data obtained in evaporimeters USWB, Type A using relative fresh water.

Go to Fig.1

Groundwater Evaporation through the soil column
This process occurs when the water table is relative shallow. Depending on the depth of the water table, evaporation can occur under:

1. Wet conditions: When the water table is close to the surface, the soil between these boundaries will be wet enough that evaporation will only be controlled by the external meteorological conditions, independently of the physical properties of the soil (It means, the rate of water supply to the evaporation front is greater than the rate of evaporation).

2. Dry Conditions: If the water table is deep enough, the shallow soil will become quite dry. In this case, the rate under which the unsaturated soil profile can transmit water toward the surface is less than the atmospheric evaporative potential, been this the limiting condition. The evaporation rate then will be determine exclusively by the capacity of the unsaturated soil to transport humidity, and the evaporative surface will be located within the soil profile.

Based on the previous ideas and assuming the evaporation process occurred under steady state conditions, the maximum humidity flux that the soil can transmit will be determine by the location of the water table and the hydraulic properties of porous media. Grilli and Vidal (1986) proposed the following relationships:

Evap = Eb when Z < zlím, and
Evap = Eb exp[-a (Z –Zlím)] for Z > Zlím

Where “Evap” represents the soil evaporation when the saturated level is located at a depth “Z” below the soil surface,“Eb” is the evaporation rate corresponding to the free surface of the brines (exposed to the surface). This condition is reached when the groundwater is located at a depth less or equal than“Zlím” and “a” is a constant parameter.

Other relationship Evap = Eo Z-m was proposed by Mermoud and Morel-Seytoux (1989) to evaluate evaporation, where “Z” is the depth to the water table, and “Eo” and “m” are dependent constants of the hydraulic properties of the porous media.

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Go to Fig.3

Results
Different methodologies were used in our study to evaluate the total evaporation flux in the principals closed basins located in the I, II and III regions in Chile. These methodologies are the following:

1. Water Balance: The evaporative volume in the terminal zone is calculated based on the affluent volume to the zone of high evaporation plus the precipitation over the evaporative surface. Furthermore, the affluent volume in this zone corresponds to the precipitation minus the real evapotranspiration in the affluent subbasin to the evaporation zone.

2. Lysimeters: These instrumentations consist of buried tanks open only on the surface of the soil. In these tanks, that contain unaltered soils, the water level can be controlled and it is possible to measure directly the water lost by evaporation.

3. Theory of flux through the unsaturated zone: Based on this theory, it is possible to calculate the ascendent volume using the gradients that generate the humidity movement and through the estimation of the related phenological coefficients.

4. Theory of isotope fractionation: The water molecule that ascends through the soil is affected by isotope fractionation during evaporation. This process can be modeled and evaporation can be evaluated through the application of the model to the 18O of the water obtained in profiles in the unsaturated zone, .

Go to Fig.4

The last three methodologies are applied at the local level and its extrapolation can be done using the equations proposed by Grilli and Vidal (1986) and Meermoud and Morel-Seytoux (1989).

Our study, whose main objective was to validate techniques and instrumentation at field conditions to estimate evaporation, made possible the evaluation of the average annual amount of water resources that are lost by evaporation in the main closed basins in Northern Chile. These results are presented in Table 1. Go to Table 1

Conclusions and Recommendations
The results obtained in our study for the different regions in Chile are presented on Table. 2. Our results showed that reducing or avoiding the evaporation of the groundwater could increase significantly the amount of renewable water resources available for exploitation. Go to Table 2

It is possible to use a fraction of the water volume that is lost by evaporation intercepting the volume of groundwater reaching the zone of high evaporation. This interception that can be done using pumping wells that will deepen the water table in the aquifer with the effect of reducing the amount of water lost by evaporation. Then, the renewable volume that could be utilized can be calculated as the difference between the volume actually evaporated and the volume that should be evaporated under the new location of the water table.

However the use of this water could created some problems that need to be addressed before implementing this alternative. First, due to pumping the aquifer can be contaminated with saline water present in the areas of high evaporation. Secondly, the volume of surface water can be reduced affecting the water level in the lagoons that are essential for wild life in these arid environments. This effect can be minimized replenishing the surface water with groundwater, obtained using pumping wells, with the same chemical characteristic than the springs feeding the lagoons. This mitigation practice has to continue until the original volume of the springs is recovered even after no more water is used for human activities (Mining, agriculture and cities) . Thirdly, it could be an impact in the amount of water available for the natural vegetation. This impact can be handle using artificial irrigation using the groundwater pumped from the mitigation wells.

References

Grilli, A y Vidal, F. (1986) “Balance Hidrológico Nacional. II Región”. Dirección General de Aguas, Pub. SDEH 86/1

Ide, F. (1978) “Cubicación del Yacimiento Salar de Atacama. Corfo, Comité de Sales Mixtas.

Mermoud, A. y Morel-Seytoux H.J. (1989) “Modélication et observation du flux hydrique vers la surface du sol depuis une nappe peu profonde. Hydrol.Continent. Vol.4, Nº1, pp.11-23 ORSTOM

Warren, C (1966) “Factors Determining the Rate of Solar Evaporation in the Production of Salt. 2º Ohio Geol.Soc.Symp.of Salt. Pp.152-167