Predictions of the Soil Water Flux Based upon Field-measured Soil-water Properties
1977
Warrick, A. W. | Mullen, G. J. | Nielsen, D. R.
Predictions of soil water flux are made taking into account spatial variability of the soil water parameters. A conductivity of the form K = Kₒ exp [α(θ − θₒ)] is utilized where θ is the volumetric water content with Kₒ and θg the values of hydraulic conductivity and water content for steady-state intake from water ponded on the soil surface. The value of α is an empirical constant for each site and for each depth. Monte Carlo simulations are used to simulate the flux distribution from the simplified drainage equation JL = Kₒ (1 + α Kₒ t/L)⁻¹. The flux of JL is a random output dependent upon the stochastic nature of Kₒ and α. Long-normal distributions of Kₒ and α are chosen from field data published for Panoche soil. Output of JL is approximately lognormal for all times studied. The mean value of JL is larger than the flux calculated using the average Kₒ and α values in the above equation. Sample numbers necessary to estimate the means of the flux at t = 0, 1, and 10 days are plotted. For t = 0, samples of 1,000, 100, and 50 result in estimates of JL within 16, 50, and 60% of the true mean 95% of the time. For t = 1 and 10 days similar figures for samples of 1,000, 100, and 50 are 4, 14, and 20%. Values of water content stored in the profile using the above equations are also examined as a random function. Unlike the flux values the range in values stays about the same for at least 20 days. The research points out a need for meaningful methods for sample stratification in order to reduce variances.
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