10.2: Evaporation from a water table

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Page ID: 38808

Tyson Oschner
Coalinga College

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In locations with a relatively shallow water table, evaporation from the soil can proceed for long periods of time and can dramatically impact the water balance and solute transport. In fact, evaporation from soils with shallow water tables is one of the major factors in the global crisis of soil salinization. When water is transported to the soil surface and evaporates, it leaves behind the salts it carried and those salts accumulate over time (Fig. 10‑2).

image of soil salinization — Figure 10‑2. Soil salinization on rangeland in Colorado, USA. A layer of salt covers the soil surface and coats the wooden fence post. Public domain image. Source [website]

Salinization is the accumulation of salts in the soil to a level that negatively impacts agricultural production, ecosystem health, and economic welfare [1]. Soil salinization contributed to the downfall of ancient societies in Mesopotamia [2], and it currently affects approximately 397 Mha worldwide or 3.1% of Earth’s land area [3]. Understanding the physics of evaporation from a shallow water table can help us better understand the related process of salinization.

In the simplest approximation, we can treat evaporation from a shallow water table as a steady-state process, meaning that the rate of water movement is assumed to be constant over time and the soil is neither drying nor wetting. All the soil water that evaporates is assumed to be replenished by upward flow from the underlying groundwater table. To estimate the rate of evaporation in this case, we will apply the Buckingham-Darcy Law, Equation 4-5.

$q=-K(\theta)\frac{d(\Psi_p+\Psi_g)}{dz}$

(Equation 10-1)

To apply the Buckingham-Darcy Law in this case, we define using Campbell’s hydraulic conductivity function written in terms of pressure potential:

$\frac{K(\theta)}{K_s}=\left(\frac{\Psi_e}{\Psi_p}\right)^{2+3/b}$

(Equation 10-2)

where ψ_e is the soil’s air-entry potential. We define z as positive down and the water table depth as L, and we assume the pressure potential at the soil surface is -∞, allowing us to integrate Equation 10-1 to find the maximum possible steady evaporation rate, E_max, for a given soil and water table depth. The resulting equation is:

$E_m_a_x=K_s\left[ \frac{-\pi \Psi_e}{LNsin(\pi/N)}\right]^N$

(Equation 10-3)

where N = 2 + 3/b. To see how we arrive at this equation and to see an example of how it can be applied, please watch this [website] and follow along with paper, pencil, and calculator, working the example. The figure below shows the maximum steady evaporation rate calculated using (Equation 10-3) for a silty clay, a sandy loam, and a loamy sand for water table depths from 0.5-3 m. The b and ψ_e values were taken from Table 3-2, and the Ks values were 37 mm d-1 for the silty clay, 1020 mm d-1 for the sandy loam, and 3030 mm d-1 for the loamy sand [4]. For any water table depth, the lowest evaporation rates are predicted for the coarsest- textured soil in this example, the loamy sand. However, the highest evaporation rates are predicted for the sandy loam rather than the silty clay, which is the finest- textured soil in this example. This indicates that evaporation from a water table, and associated salinization concerns, may be greater for some medium- textured soils compared to coarser or finer - textured soils. The predictions of Equation 10 - 3 become unrealistically high as the water table depth approaches the soil surface , at which evaporative demand may become the main factor limiting evaporation.

Fig. 10-3.png — Figure 10-3. Maximum steady state evaporation rate for silty clay, sandy loam, and loamy sand as a function of water table depth

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