D. R. Nielsen

Irrigation of agricultural crops

Irrigation of agricultural crops , Irrigation of agricultural crops , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

Air entrapment effects on infiltration rate and flow instability

Experiments were conducted to quantify the effects of entrapped air on water infiltration into a loamy sand. Transparent three-dimensional (3-D) and 2-D columns were used for experiments carried out for two infiltration conditions: (1) when air was free to move ahead of the wetting front and leave the bottom of the column (air draining) and (2) when air was confined ahead of the wetting front and hence could escape only through the soil surface (air confining). The measurement setup was composed of a tension- pressure infiltrometer, an air flowmeter, water manometers, and video-picture cameras. We applied both positive and negative water pressures at the soil surface and measured the simultaneous changes in the rates of water inflow and air outflow, the air pressure ahead of the wetting front, and the dynamic behavior and advance of the wetting front. The air pressure ahead of the wetting front for the air-confining condition was generally found to increase with time rather than reaching a constant level, as observed in other studies by other researchers. The air pressure fluctuated locally because of air escaping from the soil surface. On the basis of an analysis of the results we present two empirical equations to predict the maximum air pressure at which air begins to erupt from the soil surface and to predict the minimum air pressure at which air eruption stops. We found that the infiltration rate was always equal to, and controlled by, the rate of air outflow. The infiltration rate varied inversely with the air pressure ahead of the wetting front and with the ponding depth at the soil surface. The infiltration rate fluctuated with time rather than undergoing changes in a three-stage process, as is often characterized in the literature. The volume of residual entrapped air in the air-confining condition increased 7% on average, and the infiltration rate decreased threefold to tenfold as compared to the air-draining condition. Finally, it was shown that the air-confining infiltration flow is fingered and unstable, consistent with the predictions of an existing theory.

Software to model soil water retention curves (SWRC, version 2.00)

A software for the adjustment of soil water retention curves (SWRC) is presented, using twelve models found in the literature.

Two-phase flow infiltration equations accounting for air entrapment effects

Water infiltration into the unsaturated zone is potentially affected by air compression ahead of the wetting front. Analytical infiltration equations accounting for air compression, air counterflow, and flow hysteresis in a porous medium were derived on the basis of the Green and Ampt [1911] assumptions. Air compression ahead of the wetting front was predicted using the perfect gas law. The capillary pressure at the wetting front was found to vary between the dynamic water-bubbling value and the dynamic air-bubbling value of the material. These equations, accounting also for the effects of macropores near the soil surface, turned out to be simpler than the traditional Kostiakov [1932] and the Philip [1957a, b, c, d] equations. The equation parameters are physically meaningful and can be readily obtained from field measurements of the natural saturated hydraulic conductivity and soil water retention or pressure infiltrometer data. Experimental testing showed that the equations are reasonably accurate.

Spatio-temporal patterns and covariance structures of soil water status in two Northeast-German field sites

Abstract Spatio-temporal patterns of soil moisture status highly affect the heterogeneity of soil water and solute transport and leaching of chemicals to the groundwater. In order to quantify and describe spatial variability of ecologically highly relevant spatial and temporal processes linked to soil moisture at the land surface, the spatio-temporal covariance structure and the reasons for its change in time need to be identified. Therefore, soil water pressure head was monitored in two surface horizons between April and November 1995 at two field sites with a shallow ground water table, a sandy loam and a heavy clay soil in north-east Germany. For the 10- and the 30-cm depth of the sandy loam soil and for the 30-cm depth of the heavy clay soil the variance of soil water pressure head h (on log basis) was large under wet conditions. With decreasing soil water pressure head the variance of log 10 (− h ) decreased to a critical value, for which a spatial correlation structure disappeared. With further drying, the variance of log 10 (− h ) increased again, and a spatial range of correlation existed. During drying, temporally stable variation patterns developed at both field sites. The change of variance of log 10 (− h ) which is probably associated with changing degree of heterogeneity of flow conditions validates the findings of Roth (1995) obtained from stochastic flow model calculations. In general, the temporal correlation length was better defined than that of spatial correlation. At both depths, correlation lengths of the sandy loam were larger than those of the heavy clay soil. Random spatial variation of log 10 (− h ) occurred under conditions when the hydraulic gradient was close to zero. With our experimental design we could identify (i) temporal persistence of spatial patterns and correlation ranges of soil water status that can be used for representativity studies, (ii) the within-site variation of land surface moisture status, and (iii) a basis for description of spatial processes of effective soil properties that are linked to soil water status at the land surface.

Physical basis for a time series model of soil water content

A first-order autoregressive Markovian model AR(1) is formulated on the basis of the hydrologic budget and soil water transport equation. The model predictions compared well with neutron probe measurements of soil moisture content, and the statistical moments were conserved. The applied water events were white noise in structure, and the random shocks generated from the flow dynamics simplifications have a statistical mean of zero and were uncorrelated for all time lags. The derived AR(1) model parameter is used to compute the mean diffusivity of the soil, which is in agreement with reported lab measurements and field estimates obtained from cumulative evaporation measurements made with two large lysimeters.

Simultaneous scaling of soil water retention and hydraulic conductivity curves

Previously, soil water retention and hydraulic conductivity curves have been scaled independently with separate scale factors calculated for each curve. Here we provide a method to scale both hydraulic functions simultaneously to yield a single set of scaling factors. An algorithm was developed to scale the hydraulic data, either as a function of degree of saturation S, or normalized water content Θ. Optimization using Powells method in combination with a Newton-Raphson procedure resulted in one set of parameters representing the scaled mean hydraulic functions and one set of scale factors describing the variability of both the retention and conductivity relationships. We present and compare results obtained from (1) scaling water retention data only; (2) scaling hydraulic conductivity data only; (3) simultaneous scaling of soil water pressure head h and the natural logarithm of hydraulic conductivity K; and (4) simultaneous scaling of the logarithms of both h and K. Sets of scale factors for two data sets were analyzed for distribution type and autocorrelation.

Estimation of in situ hydraulic conductivity function from nonlinear filtering theory

Note: 29(4): 1063-1070 Reference EFLUM-ARTICLE-1993-006doi:10.1029/92WR02593 Record created on 2005-09-08, modified on 2017-02-23

Water movement in glass bead porous media: 2. Experiments of infiltration and finger flow

This paper presents experimental observations of infiltration and finger flow in glass beads. In paper 1 (Lu et al., this issue), we showed that the total surface tensile force is much greater in initially wet profiles than in initially dry profiles. During capillary rise in glass beads, the “jump” process takes place for an initially dry condition, whereas in an initially wet profile not only a jump process but a film thickening associated with film flow characterizes capillary rise. In this paper, infiltration experiments into initially dry glass beads show that the wetting front is relatively saturated and flat compared with the unsaturated and irregular wetting front into an initially wet profile. In the experiments of finger flow, photographs show that the tip of the finger is completely water saturated and that no partially saturated zones exist around the saturated tip. The fingers initiated in a dry zone disappear when they reach an initially wet lower zone even when the packing conditions of the glass beads are identical. Hence the criterion for instability when water is applied at a rate less than the value of the saturated hydraulic conductivity does not apply to an initially wet condition. When a fine layer of glass beads lies on a coarse layer that is initially dry, fingering will take place during infiltration and flow is unstable. If the coarse lower layer is initially wet, finger flow does not develop, and the flow remains stable. More investigations are required to ascertain threshold values of the initial water content causing instability of water movement in porous media.

Scaling of semivariograms and the kriging estimation of field-measured properties

Two methods were evaluated for scaling a set of semivariograms into a unified function for kriging estimation of field-measured properties. Scaling is performed using sample variances and sills of individual semivariograms as scale factors. Theoretical developments show that kriging weights are independent of the scaling factor which appears simply as a constant multiplying both sides of the kriging equations. The scaling techniques were applied to four sets of semivariograms representing spatial scales of 30 x 30 m to 600 x 900 km. Experimental semivariograms in each set successfully coalesced into a single curve by variances and sills of individual semivariograms. To evaluate the scaling techniques, kriged estimates derived from scaled semivariogram models were compared with those derived from unscaled models. Differences in kriged estimates of the order of 5% were found for the cases in which the scaling technique was not successful in coalescing the individual semivariograms, which also means that the spatial variability of these properties is different. The proposed scaling techniques enhance interpretation of semivariograms when a variety of measurements are made at the same location. They also reduce computational times for kriging estimations because kriging weights only need to be calculated for one variable. Weights remain unchanged for all other variables in the data set whose semivariograms are scaled.