摘要:

Tension infiltrometers have become popular for determining unsaturated hydraulic conductivity (K) and associated measures in situ. The objective of this study was to use tension infiltrometers to determine K, sorptivity (S), macroscopic capillary length (Λc—a characteristic negative head), and gravity time (tgrav) several times over a growing season for different negative heads and infiltrometer base sizes. Infiltration was measured on seven dates, within 20 m. on a Webster clay loam (Fine, mixed, mesic, Typic Haplaquoll). The soil had been disked before planting corn (Zea maysL.) and cultivated twice after planting. All measurements were made in nonwheel-tracked interrows. Infiltration was measured for ascending heads (-150, −60, and −30 mm) at the same location. Both K and S fluctuated over the growing season with no relation to initial soil water content, which varied from 0.04 to 0.36 m/m. Over the growing season, large-base infiltrometers consistently had smaller calculated Λc(80 mm) than small-base infiltrometers (182 mm), which indicated more influence of gravity for large-base infiltrometers and more influence of capillarity for small base. Pooled across dates, there was no significant difference in base size for S or K. Gravity time is a crude estimate of the time necessary to reach steady-state, and calculated tgravwas usually less than 1 h. Tension infiltration indirectly indicates pore arrangement, suggesting that pore arrangements varied over a growing season.

ISSN:0038-075X    

出版商:OVID    

年代:1993

数据来源: OVID

摘要:

The Gregson et al. one-parameter model (GHM) is based on the log-log form of the soil water retention curve, below the air-entry value of ψ, ln ψ =a+bln &thetas;, whereaandbare the intercept and slope, respectively. A strong linear relationship observed betweenaandbwas expressed asa=p+qb. Given this relationship, the GHM was derived as ln ψ =p+b(ln &thetas; +q). Givenpandqvalues for a soil or group of soils, only one value of the ψ(&thetas;) relationship needs to be known to calculate the only unknown parameter in the model - b, and hence, the entire ψ(&thetas;) function. Typically, &thetas; at the −33 kPa matric potential (&thetas;-33 kPa) is used as the known ψ(&thetas;) value. Here we provide a regression relationship betweenband the available water content (AWC) to estimateb, since in many cases the AWC is available in the USDA soil survey reports, whereas &thetas;-33 kPais not. Using thebthus estimated in GHM gives only slightly larger errors in calculating the water content at different potentials than when using &thetas;-33 kPa. Further we show that the intercept (a‘) and slope (b’) of a log-linear model, ln ψ =a‘+b’&thetas;, are also linearly related and an alternate form of the one-parameter model (LLM) can be derived, ln ψ, =p‘+b’(&thetas; +q‘), which uses AWC directly. The errors with this model are comparable to GHM. Unfortunately, LLM requires individual soilp’andq‘values and, because of more scatter in the intercept - slope relationship, pooledp’andq‘values for a group of soils are not as effective in LLM as they are in GHM.

ISSN:0038-075X    

出版商:OVID    

年代:1993

数据来源: OVID

摘要:

When solving water flow in a soil using Darcy-Richards' equation, information about the unsaturated hydraulic conductivity of the soil is necessary. However, direct measurement of unsaturated hydraulic conductivity is time consuming and expensive. Therefore, instantaneous profile methods and/or models to estimate the unsaturated hydraulic conductivity from water retention data are preferable. However, results of the instantaneous profile methods are often influenced by how precise we express the relation of measured volume wetness with time and the relation of measured hydraulic heads with soil depth. Here, we introduce three regression functions for expressing these relationships. In addition, an empirical model relating the measured unsaturated hydraulic conductivity with volume wetness was also developed. Evaluations of Burdine and Mualem models to predict the unsaturated hydraulic conductivity from the water retention curves were conducted. The results show Burdine and Mualem models can estimate the unsaturated hydraulic conductivity better for the tested soils at lower volume wetness.

ISSN:0038-075X    

出版商:OVID    

年代:1993

数据来源: OVID

摘要:

The zero flux plane (ZFP) method for estimating actual evapotranspiration (ET), derived from the work of Richards et al. (1956. Soil Sci. Soc. Am. Proc. 20:310–314) and Arya et al. (1975. Soil Sci. Soc. Am. Proc. 39:424–430), was field tested as suggested by M. L. Sharma (1985. Academic Press, New York). The ZFP method requires total hydraulic head and soil water content data from the upper few feet of a soil. The total hydraulic head data, derived from tensiometers, is used to identify the “zero flux plane.” This plane separates the region of the soil where water flow is upward from the region where flow is downward. When the ZFP is below the root zone, only water above the zero flux plane is available for ET. Combining this knowledge with water content data allows estimation, using an equation given here that is based on information from Arya et al. (1975), of actual water lost from the region above the ZFP for the period of interest.Where applicable, several advantages make the ZFP method theoretically more attractive than other techniques for estimating ET: (i) it is based on measurements made directly in the controlling medium, the soil; (ii) it intrinsically integrates the effect of atmospheric, soil and plant processes which affect water movement in the soil and hence ET; and (iii) it is simple, relatively inexpensive, and conceptually sound.

ISSN:0038-075X    

出版商:OVID    

年代:1993

数据来源: OVID

摘要:

Rainfall intensity (I), antecedent water content, and slope steepness affect soil loss from interrill areas. Objectives of this field study were to (i) test the validity of the assumption that interrill soil loss is a function ofI2, (ii) determine effect of antecedent water content on interrill erodibilities (Ki) for two slope steepnesses, (iii) evaluate the slope factor used in the water erosion prediction project (WEPP), and (iv) evaluate the product of I, flow discharge, and slope steepness as a possible interrill erosion model. Four soils, ranging in texture from sandy loam to clay, were exposed to sequences of simulated rainfall at three intensities (50, 70, and 100 mmh−1). Soil loss and runoff were measured from air-dried and prewetted flat plots and from air-dried and prewetted ridged plots. For each plot type (slope steepness),bvalues forE=albvaried between air-dried and prewetted conditions with no apparent trend. Values of b ranged between 0.91 and 2.05, although mostbvalues did not approach 2.0. Prewetting had no effect on Heiden clay and Cecil sandy loamKivalues for the modelE=KiI2, but it decreasedKivalues for Miami silt loam and Broughton clay by at least 21 and 31%, respectively. Increased slope steepness associated with ridged plots reduced the degree of surface sealing as indicated by decreased soil strength. Values ofKiforE/Sf=KiI2, whereSf= WEPP slope factor, were different for different slopes.Sfwas a function of soil type, antecedent water content, and individual interrill erosion processes controlling sediment delivery.

ISSN:0038-075X    

出版商:OVID    

年代:1993

数据来源: OVID

摘要:

Shrinkage and cracking characteristics of swelling clays at two sites under border strip irrigation were examined. At one site, irrigation water salinity was varied. Image analysis of soil surface photographs was used to quantify surface ped geometry. Volumetric surface soil samples were used to find temporal and spatial variation of water content and bulk density both on the surface and with depth. Qualitative observations of cracking behavior were made in the field.There was no spatial dependence for water content and bulk density at the scale of several centimeters or between peds. Larger, more polygonal peds were observed in higher salinity treatments, wheel traffic rows, and after repeated irrigations. Surface shrinkage increased with salinity. Dry bulk density was higher on the high salinity treatment both on the surface and with depth. Despite larger area, perimeters of the 9000 ppm treatment peds were less than perimeters for other treatments. Surface percent crack area followed percent change in bulk density.

ISSN:0038-075X    

出版商:OVID    

年代:1993

数据来源: OVID

摘要:

Depth, width, and length per unit area of cracks were measured in a Udic Haplustert from India during the summer season after the harvest of different crops (chickpea, wheat, sorghum, sunflower, and safflower) and compared with that of a fallow plot. The width, depth and length of cracks exhibited a wide size range. Crack width was significantly less (1.3 cm) in the fallow plot compared with those plots planted to crops. Crack depth in a majority of the plots was not significantly different from fallow except for plots planted to safflower and sorghum. Crack length was the highest in the fallow plot compared with cropped plots and, as a result, the surface cracking pattern was more intricate in the fallow plot. No single crack dimension adequately characterized the cracking intensity. Hence, crack volume per unit area was considered to be a better index of cracking intensity because it includes all three crack dimensions. Crack volume computed by measuring length, depth, and width, and that measured directly by infilling with sand, approximated each other. The crack volume was highest in plots planted with safflower (445 m3ha−3) because of its deep rooting and efficient moisture extraction and least in the fallow plot (234 m3ha−3). In other cropped plots the values were intermediate. The crack space assumes significance in that it can accommodate 2344 min of rainfall facilitating moisture recharge in these otherwise slowly permeable soils.

ISSN:0038-075X    

出版商:OVID    

年代:1993

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1993

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1993

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1993

数据来源: OVID