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Irrigating Forage Crops with Saline Waters

2. Modeling Root Uptake and Drainage

George E. Brown, Jr., Salinity Lab., USDA-ARS, 450 W. Big Springs Rd., Riverside, CA 92507

View larger version (19K): [in a new window]   Fig. 1. Relationship between solution salt concentration c (1 meq L–1 = 1 mmolc L–1), osmotic pressure head h, and electrical conductivity EC. The plotted data are EC measurements made on the irrigation waters used in Skaggs et al. (2006) and UNSATCHEM (imnek et al., 1996) computations of EC and h based on the composition of the same waters. The solid lines are fits to the data. A common dilute-solution approximation, EC = 0.1c, is shown for reference (dashed line).

View larger version (20K): [in a new window]   Fig. 2. Comparison of model simulation and data using final hydraulic parameter estimates.

View larger version (46K): [in a new window]   Fig. 3. Measured and simulated cumulative drainage for Phase 1 experimental treatments in which all lysimeters where luxuriously irrigated with various irrigation water salinities (ECiw). Irrigation timings are shown on the bottom of each plot and the irrigation depth is indicated on the axes on the right side of each plot.

View larger version (33K): [in a new window]   Fig. 4. Measured and simulated drainage electrical conductivity for Phase 1 experimental treatments using various irrigation water salinities (ECiw). The open circles in each plot are measured daily averages and the solid lines are the simulated drainage EC. The horizontal line in each plot is a steady-state approximation (see Eq. [17]). Drainage was not continuous in the simulations, leading to the broken appearance of the solid lines. The irrigation for each lysimeter is shown in Fig. 3.

View larger version (35K): [in a new window]   Fig. 5. Measured and simulated cumulative drainage for alfalfa lysimeters during Phase 2 experimental treatments using various irrigation water salinities (ECiw) and target irrigation depths f. Irrigation timings are shown on the bottom of each plot and the irrigation depth is indicated on the axes on the right side of each plot.

View larger version (35K): [in a new window]   Fig. 6. Measured and simulated cumulative drainage for tall wheatgrass lysimeters during Phase 2 experimental treatments using various irrigation water salinities (ECiw) and target irrigation depths f. Irrigation timings are shown on the bottom of each plot and the irrigation depth is indicated on the axes on the right side of each plot.

View larger version (27K): [in a new window]   Fig. 7. Measured and simulated drainage electrical conductivity for alfalfa lysimeters during Phase 2 experimental treatments using various irrigation water salinities (ECiw) and target irrigation depths f. The open circles in each plot are measured daily averages and the solid lines are the simulated drainage EC. The simulated and measured drainage EC appears broken or missing in some plots because drainage did not occur or occurred infrequently in some lysimeters (see Fig. 5).

View larger version (27K): [in a new window]   Fig. 8. Measured and simulated drainage electrical conductivity for tall wheatgrass lysimeters during Phase 2 experimental treatments using various irrigation water salinities (ECiw) and target irrigation depths f. The open circles in each plot are measured daily averages and the solid lines are the simulated drainage EC. The simulated and measured drainage EC appears broken or missing in some plots because drainage did not occur or occurred infrequently in some lysimeters (see Fig. 6).

View larger version (27K): [in a new window]   Fig. 9. Comparison of uptake reduction functions used in the simulations with functions derived from published plant salt tolerances.

View larger version (26K): [in a new window]   Fig. 10. Comparison of measured cumulative drainage with cumulative drainage simulated using uptake reduction functions derived from Maas (1990) salt-tolerance data. Electrical conductivity of the irrigation water (ECiw) was 13 dS m–1 and the target irrigation depth f was 1.25 for both crops.