# Evaporation and Salt Precipitation in Soil Cracks: Numeric Analysis Christopher Brian Graham1 Maria Ines Dragila1, Noam Weisbrod2 (1) Department of Crop & Soil Sciences, Oregon State University, USA; (2) Department of Environmental Hydrology & Microbiology, Institute for Water Sciences & Technologies, BIDR, Ben-Gurion University of the Negev, Israel Physical Model Effect of fracture on evaporation (no salt) A conceptual model of rock fracture and soil crack air convection has been developed by Dragila and Weisbrod. The conceptual model describes increased evaporation from crack surfaces due to flushing of “wet” crack air with “dry” surface air. Increased evaporation leads to increased salinization of the near crack soil matrix. The conceptual model is being tested with laboratory and field experiments and measurements. Further testing is being done using numeric methods. Based on soil crack and lysimeter described in Ritchie and Adams (1972). Lysimeter is filled with Houston Black Clay Soil lysimeter has cross sectional area of 1.83 m x 1.83 m and depth of 1.2 m. Crack is 50 cm deep, with surface width of 5 cm, running from corner to corner of lysimeter. The fracture evaporative front is seen to increase cumulative evaporation by 20% over 100 day simulation (figure x). Effect of fracture is greatest at early time, with evaporation rates converging towards end of simulations. Fracture lowers soil water content at all depths after 100 days. (figure x) 0 0 5 10 15 Motivation Conceptual model describes increased water loss due to crack evaporation, with effects on water balance calculations, plant water stress and other concerns. Increased salinization due to crack evaporation can lead to salinization issues at the surface and subsurface, caused by crack preferential flow to aquifer. -0.1 -0.15 -0.15 Depth (m) Depth (m) Figure 1: Crack air convection due to thermal instability (left) and schematic of convection enhanced evaporation (right) 20 -0.2 -0.25 -0.3 -0.35 -0.2 -0.35 -0.4 -0.45 0.1 0.15 0.2 15 20 -0.5 0 0.25 20 -0.3 -0.4 0.05 15 -0.25 -0.45 -0.5 0 10 -0.05 -0.1 5 -0.05 Figure 1: Large scale representation of evaporation from a cracked (left) and uncracked system. Depth (m) Depth (m) Picture of air convection 0.05 Distance from fracture (m) Distance from crack (m) 0.1 0.15 0.2 0.25 Distance from fracture (m) Distance from crack (m) Figure 1: Water content profile in uncracked system Cumulative Evaporation (kg) Cumulative Evaporation (kg) Conceptual Model 2.5 Surface Only Crack and Surface 2 1.5 1 0.5 0 0 20 40 60 80 T ime (days) Time (days) Figure 1: Water content profile in cracked system Figure 1: Cumulative evaporation for cracked and uncracked systems Objectives Develop numeric model of evaporation from soil cracks using TOUGH2, an multiphase multicomponent porous media simulator. Use model to determine extent and effect of evaporation in soil cracks on water balance and salt redistribution. Experimental Water Mass Simulated Water Mass 380 360 340 320 300 280 260 240 220 0 10 20 30 40 50 60 70 80 Time (days) Time (days) Figure 1: Experimental Setup Figure 1: Numeric grid of representative slice. Element size is reduced near upper and fracture boundary to increase evaporation modeling precision Numeric Model Figure 1: Simulation and experimental results Entire lysimeter is modeled using a representative 5 cm slice, 60 wide and 120 cm deep. Hydraulic properties of Houston Black Clay are modeled using the van Genuchten curves using parameters for clay from Carsel and Parrish. Evaporation rate is set at 1.4 mm/day, matching average value from Ritchie and Adams (1972). Lower boundary is constant head, allowing for gravity drainage, right and left boundaries are no flow, for symmetry. Upper boundary and fracture are constant head, representing a well mixed atmosphere. 4 4 Total Salt Aqueous Salt Solid Salt 3.5 Normalized salt content Figure 1: Physical model of weighing lysimeter. 5 cm slice represents entire lysimeter due to symmetry. Full depth is modeled. Evaporation From Sand Column, 80 Days 400 Mass water (kg) Water mass (kg) While EWASG module of TOUGH2 has been shown to accurately model saturated and unsaturated water and salt flux, and solid phase salt precipitation, its capabilities regarding evaporation are untested. A small scale evaporation experiment was constructed and simulated using the EWASG module of TOUGH2. A hydrometer cylinder was filled with saturated sand and allowed to evaporate for 80 days. Simulated results closely matched experimental data with minor modifications to the standard modeling procedures. Evaporation from soil crack is seen to increase salt content near fracture by 3 to 4 times. Salt content increases to the point of salt precipitation along fracture surface (figure x). 3 2.5 Simulations 4 Simulations are run using geometric framework, with and without salt 1. 100 day simulation without fracture, no salt 2. 100 day simulation with fracture, no salt 3. 100 day simulation with fracture and low salt content (15 g salt/kg solution) 4. 100 day simulation with fracture and high salt content (100 g salt/kg solution) 2 1.5 1 0.5 0 0 2 4 6 8 Total Salt Aqueous Salt Solid Salt 3.5 3 2.5 2 1.5 0 20 1 40 60 0.5 80 0 0 10 2 Distance from crack (cm) 4 6 8 10 Depth below surface (cm) 100 Figure 1: Salt concentration normalized by initial salt concentration, high initial salt concentration simulation. Figure 1: Salt concentration normalized by initial salt concentration, low initial salt concentration simulation. Figure 1: Solid phase salt precipitation distribution in soil matrix after 100 days 2.5 Case Study: Alvord Desert, Oregon Conclusions Implications Evaporation is enhanced by soil cracking (an increase of 20%) While not as great an increase as previously reported (Ritchie and Adams, 1972), this increase can have an impact on water balance calculations, irrigation applications and salinization control. A larger impact is seen in salt redistribution. Salt levels increase 3-4 times near crack. Soil structure (cracks) must be taken into account when calculating a water balance or planning irrigation. Management tools may be able to reduce water loss through evaporation by 20% Experimental results suggest that the majority of salt concentration occurs as precipitated salt in the crack void, which leaves it susceptible to flushing and aquifer salinization during rain events. Shallow (< 1m) lake fills desert in winter and early spring, draining and evaporating through summer. Lake dimensions: 12 x 7 miles or 21,760 hectares. Without soil cracks, assuming clay soil and potential evaporation rate of 1.4 mm/day, can expect evaporation of 0.26 mm/day 4,530 acre feet of water over course of summer. With soil cracks, assuming a crack density of 1 crack/m2, expect an increase of 20%, or over 900 acre feet of water. Evaporation from cracks can be expected to increase soil salinity 3-4 times near cracks. Assuming a low salt content of 15 g salt/kg solution, can expect accumulation of 139,000 tons of salt in crack voids. Assuming a high concentration (100g/kg) can expect accumulation of 787,000 tons of salt in crack voids. This salt is expected to be flushed to aquifer during rain events Figure 1: Alvord Desert Figure 1: Alvord Desert in winter in summer Effect of salt on evaporation Increased salt content lowers evaporative flux at all time in three simulations. Increasing salt content leads to decreased evaporative flux Distance from crack (cm) Distance from crack (cm) Cumulative Evaporation (kg) Cumulative evaporation (kg) Validation Effect of fracture on salt redistribution Normalized salt content Figure 1: Cracked soil (left) and evaporation driven solid phase salt precipitation on saturated sandstone (right) References 2 1.5 Carsel and Parish 1 Pruess 0.5 No Salt Low Salt Lotsa Salt 0 0 20 40 60 Time (days) Time (days) 80 Ritchie and Adams 100 Figure 1: Cumulative evaporation for high, low and no salt concentrations Weisbrod and Dragila van Genuchten 100
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