Field Soil Water Content Research Articles

Nitrogen mineralization in native cultivated and abandoned fields in shortgrass steppe

We conducted a set of in situ incubations to evaluate patterns of N availability among dominant land uses in the shortgrass steppe region of Colorado, USA, and to assess recovery of soil fertility in abandoned fields. Replicated 30 d incubations were performed in 3 sets of native (never cultivated), abandoned (cultivated until 1937), and currently cultivated, fallow fields. Net N mineralization and the percentage of total N that was mineralized increased in the order: native, abandoned, cultivated. Higher soil water content in fallow fields is the most likely reason for greater mineralization in cultivated fields, while higher total organic C and C/N ratios in native and abandoned fields may explain differences in mineralization between these land uses. Recovery of soil organic matter in abandoned fields appears to involve accumulation of soil C and N under perennial plants, but probable methodological artifacts complicate evaluation of the role of individual plants in recovery of N availability. Higher N mineralization and turnover in cultivated fields may make them more susceptible to N losses; recovery of N cycling in abandoned fields appears to involve a return to slower N turnover and tighter N cycling similar to native shortgrass steppe.

Measurement of spatial average of field soil water content by the long heat probe method

Abstract The long heat probe method was developed and tested to measure the average soil water content under field conditions. The length of the probe was 20 cm and 5 or 10 m. The probe consists of a thermometric metal wire resistor to detect the average temperature of the probe along the heating resistor. The twin transient-state cylindrical-probe method was improved for this measurement. The results show that when the relationship between the thermal conductivity and the soil water content was linear, the method enabled to measure accurately the average soil water content both in the field and laboratory.

Comparison of neutron moisture gauges with nonnuclear methods to measure field soil water status

The neutron moisture gauge is compared with the gravimetric-core soil sampling technique, tensiometers and resistance blocks in relation to stability, Held variability, spatial dependence and number of samples needed at a given level of significance. The variance of field water content measurements with neutron moisture gauges is lower than that of the gravimetric sampling, which therefore requires 2 to 6 times as many samples as the number of measuring sites of the gauges to attain the same level of significance. The space dependence of the measurements made with the subsurface gauge varied depending on the average field soil water content. No space dependence was evident when the water content was lower than 0.2 cm³.cm-3 (50% saturation). Measurements with the tensiometers and resistance blocks manifested no spatial dependence and therefore randomly selected measuring sites can be adapted to Held research work where these methods are to be utilized. Soil water content measurements estimated with neutron moisture gauges showed well defined temporal stability (i.e., the lowest, average and the highest soil water content measurements occur at the same field site) which implies that soil water status of an entire field can be assessed with measurements limited to few locations. Measurements with both tensiometers and the resistance blocks are time variant (i.e., the site giving field average water content changes spatially in time) owing to their relatively smaller measuring domains (i.e., scale of the area which can be represented by a single measurement) as compared to neutron gauges. Therefore it is not possible to define the measuring sites of the tensiometers and resistance blocks as to assess soil water status of the entire field, as it could be done with the neutron gauge.

Core sampling technique for bulk density and porosity determination on a clay loam soil

Seasonal fluctuations in bulk density and porosity determined from soil cores have been observed to be independent of soil and cropping treatments on the clay and clay loam soils of southwestern Ontario. An experiment was conducted to determine if the fluctuations may be a result of inconsistencies in core sampling technique. Three core sampling techniques were compared on five dates on each of four long-term cropping treatments which provided a range in soil water contents and structure. The core sampling techniques compared were: (1) a typical hand-held, hammer driven, double cylinder core sampler with the recommended procedure; (2) a typical hand-held, hammer driven, double cylinder core sampler with an excess number of blows; (3) pressing the double cylinder core sampler into the soil hydraulically. Variations in core sampling technique significantly affected the observed values of bulk density, total porosity, and air-filled porosity at field soil water contents ranging from 12.6 to 23.8% by weight. However, hammering, compared with hydraulically pressing, the core sampler into the soil, appears to cause more distortion within the soil core which increases variability. The effect of sampling date, and its inherent effect on soil moisture, was generally greater than the effect of the core sampling techniques. Therefore, providing a consistent technique is used, observed seasonal fluctuations in bulk density, total porosity, and air-filled porosity on Brookston clay loam soil are apparently not the result of variations in the core sampling technique.

Water Balance Estimation Model: Field Test and Sensitivity Analysis

AbstractA simplified, capacity‐based simulation model has been developed to predict components of the field water balance. This requires only daily information such as the atmosphere evaporative demand and soil‐wetting events, maximum available soil water storage, and root growth characteristics, as well as crop and soil parameters. Actual evapotranspiration was modeled using Eagleman's parameterization. The model was tested for upland rice grown on a 3500‐m2 field plot, and on an undisturbed lysimeter. The predicted actual evapotranspiration (AET) and drainage (D) at the bottom of the soil profile matched the lysimeter data. Also, the model predicted reasonably well the field soil water content data, which were obtained at 15 sites. In the latter case, the root mean square error was about 16% for the seasonal AET, and 19% for D. Errors were within the limits of experimental uncertainties expected when such measurements are analyzed by Darcy's law. The results of a sensitivity analysis with respect to changes in parameter inputs show that the model is physically realistic. However, the sensitivity depends strongly on the supply of water in the soil. This clearly justifies the need to simulate the root‐zone water balance as a time‐dependent process, especially if there is a practical interest in accurate prediction during drought periods.

Soil Shrinkage Relationships of Texas Vertisols: II. Large Cores

AbstractAlthough soil shrinkage in the field has been related to water content changes, this relationship has not been correlated with small core measurements or defined in terms of structural and shrinkage water loss phases. Since field measurements are limited by the accuracy of the water content determination, undisturbed cores (73 cm diam and 140 cm long) from the eight Vertisols studied previously were studied to accurately measure water loss (by weighing). Sorghum plants were established, and a single drying cycle was studied. Vertical shrinkage was measured at the surface and at the 15‐, 35‐, 55‐, 95‐, and 115‐cm depths, and the water content profile was measured with neutron moisture and gamma density probes. These data were interpreted in terms of the shrinkage pattern found in the cores and with the use of small core data obtained by Yule and Ritchie. (1980).Although the water loss data were confounded by large soil evaporation from shrinkage cracks at the side of the cores, the evidence suggested that the shrinkage response was a combination of structural and shrinkage water loss phases. Consequently, the shrinkage curve for the soil surface asymptotically approached the theoretical equidimensional, normal curve. Shrinkage curves with profile depth tended to parallel the soil surface curve. The water content profiles obtained were not precise, but shrinkage parameters calculated for various depth increments resembled the small core data obtained previously. The results showed that small core data could be used to predict field shrinkage responses if field soil water content profiles are known.

Prediction of Field Soil Water Content

AbstractA 3‐year experiment was conducted to monitor and subsequently predict field water contents using the water transport model. Saturated hydraulic conductivities and soil water characteristic curve results were measured and averaged for each soil layer. The Millington‐Quirk (MQ) method was used to calculate average model parameters. Direct use of the MQ‐derived conductivity and diffusivity functions did not give an exact fit to the observed field results. However, when reasonable adjustments were made to the K(θ) and D(θ) functions the prediction was improved considerably. The methodology for “averaging” and utilizing field data to derive model parameters, and subsequent adjustment or optimization of the model parameters all require more research if we hope to use the unsaturated flow equation for field water content prediction.

MEASUREMENT OF HYDRAULIC CONDUCTIVITY AND DIFFUSIVITY FOR PREDICTING THE PROCESS OF SOIL WATER INFILTRATION FROM A TRICKLE SOURCE1

ABSTRACT: Two soil water functions, hydraulic conductivity K(θ) and diffusivity D(θ), were estimated by two methods In one method D(θ) was estimated according to Bruce and Klute (1956), and K(θ) was calculated from D(θ) and the retention curve. In the second, K(θ) was obtained by field estimation, with D(θ) being calculated from K(θ) and the retention curve.The criterion of reliability for both methods was agreement between experimental and predicted distribution of soil water content. The prediction was made using the functions K(θ) and D(θ) as soil water parameters in both methods.Theoretical and experimental agreement was generally good. The first method, however, was found to be best for high soil water content and the second for low soil water content. In addition, the water content at the end of the monotonic increase of function D(θ) (estimated according to Bruce and Klute 1956) was found to be about the upper limit of field soil water content. It can be used as a boundary condition in the numerical solution of a cylindrical model of infiltration from a trickle source. It was concluded that the best agreement between theory and experiment can be found when the combined values of D(θ) and K(θ) from both methods of estimation are used.