Temporal Stability of Spatially Measured Soil Matric Potential Probability Density Function

those fields. Both of these techniques, however, require more observations than are practical for most field manEstimation of mean water status in a field is crucial to effective agers. Efforts have been made to characterize fields irrigation water management. Problems encountered with the estimation of mean field soil water status may be attributed to spatial variabilfrom fewer observations. The application of bootstrapity of soil physical properties. Several investigators have shown tempoping techniques (Dane et al., 1986) has been used to ral stability of spatial patterns of field measured soil water content, estimate the minimum number of observations necesbut temporal stability of field measured soil matric potential (cm ), a sary for the reliable estimation of soil parameters in a measure of soil water status more appropriate for irrigation schedulvariable field. ing, has not previously been reported to last for more than a few A number of studies (Ottoni, 1984; Vachaud et al., days within one irrigation cycle. This study investigated the temporal 1985; Kachanoski and de Jong, 1988; van Wesenbeeck stability of spatial patterns of cm both within and between sequential and Kachanoski, 1988; Jaynes and Hunsaker, 1989; Gooirrigation cycles. Sixty locations in a 1-ha field were outfitted with a vaerts and Chiang, 1993; Chen et al., 1995) have shown 1-m neutron probe access tube and three tensiometers placed at that, although soil water content varies with time and 0.15-, 0.3-, and 0.5-m depths. The observations obtained from 14 d with location in the field, the pattern of spatial variabilof soil water content measurements and 46 d of cm measurements within eight irrigation cycles were analyzed with Spearman’s rank ity does not change with time when the observations correlation coefficients and a relative differencing technique. The are ranked according to the magnitude of soil water results showed temporally stable soil water content spatial patterns content or scaled against the field mean soil water conand also indicated temporally stable cm spatial patterns if assumptions tent. This phenomenon has been termed temporal stabilof full soil wetting at the beginning of the cycle and uniform evapoity. The covariants of significance in these cases were transpiration among locations were satisfied. Several locations in the determined to be primarily soil texture and topography. field estimated the field mean cm to within 10% within a given range The dependence of water content upon soil texture has of potentials, and a few estimated the field mean to within 20% across also been used to locate textural boundaries in a field the entire range of potentials tested. Other locations estimated the from measurements of soil water content along a tranlower and higher percentiles of cm with similar accuracy. sect in an irrigated field soil (Hendrickx et al., 1986). Although temporal stability has been demonstrated for soil water contents, it has not been shown for cm. I most areas of the western USA and several areas Soil matric potential is assumed to change with changes of the world, irrigated agriculture is the predominant in soil water content. However, this function is nonlinear user of water. Irrigation water is applied to crops in a and may be expected to have a spatial component of variety of ways, including surface, sprinkler, and drip variability as well (Taylor and Ashcroft, 1972; Shouse irrigation systems, with application efficiencies ranging et al., 1995). Problems encountered with the measurefrom 40 to 95%. Improvements in irrigation efficiency ment of cm using tensiometers have included the variare often possible through better irrigation system deability of measurements at a single location by hysteresign and knowledge of the soil water properties of the sis, by mechanical influences of shrinking and swelling irrigated fields. Unfortunately, determination of the soil soils, and by the effects of diurnal temperature fluctuawater properties in a given field is often complicated tions (Taylor and Ashcroft, 1972; Jackson, 1973; Warby the large spatial variability of these properties. rick et al., 1998). In spite of these measurement probSoil spatial variability has been the focus of considerlems, cm is regarded as the best measure of soil water able research during the last three decades. In addition availability for crops (Taylor, 1952, 1965; Kramer, 1983). to field studies (Nielsen et al., 1973; Greminger et al., The literature offers little information directly rele1985; Kachanoski et al., 1985; Saddiq et al., 1985; vant to temporal stability of cm in field soils. Saddiq et al. Bresler, 1989; Goovaerts and Chiang, 1993), the applica(1985) reported that variability and spatial dependence tion of geostatistics, particularly kriging and cokriging were a function of method of water application, time (Vauclin et al., 1983), and scaling theory (Simmons et after water application, and the magnitude of the mean al., 1979; Russo and Bresler, 1980; Western and Bloschl, field cm. Hendrickx and Wierenga (1990) noted that 1999) have been steps toward the characterization of temporal stability of cm persisted for only one irrigation fields on the basis of the variability of observations from interval. They proposed the use of about seven tensiomR.S. Van Pelt, USDA-ARS Plant Stress and Water Conservation eters in a given field to estimate the mean cm, but also Lab., Big Spring Field Station, 302 W I-20, Big Spring, TX 79720; advised using a value of threshold cm less negative than and P.J. Wierenga, Dep. of Soil and Water Science, Univ. of Arizona, the crop critical threshold for initiation of irrigation. In 429 Shantz Bldg. 38, Tucson, AZ 85721. The USDA prohibits discria subsequent study, Hendrickx et al. (1994) determined mination in its programs on the basis of race, color, national origin, sex, religion, age, disability, political beliefs, and marital or family that tensiometer cup size greatly influenced measurestatus.Received 8 June 2000.*Corresponding author ([email protected]. ment variability and noted that large tensiometer cups usda.gov). Abbreviations: cm, soil matric potential. Published in Soil Sci. Soc. Am. J. 65:668–677 (2001).