Research Projects

Measuring Heterogeneities of Soil Hydraulic Properties from Canopy to Interspace

PI: Todd Caldwell

Co-PI: Michael Young and Robert Nowak

Dates: September 2006 through July 2007

Funded by: NSF EPSCoR RING-TRUE III Grant Number 0447416

Keywords: ecohydrology, arid environments, soil heterogeneity

Project Description

The ecohydrology of arid environments is strongly coupled to amount and timing of available water. The surface soil structure and texture (presence/absence of fine-grained surfaces of dust accumulation or deeper restrictive soil layers of carbonate/clay deposits) affect the depth of water penetration and hence the diversity and vigor of native vegetation, rooting distributions, microbial activity and abundance, and biogeochemical processes that promote ecosystem health. Recent investigations have identified these linkages as being coupled between ecologic, hydrologic and pedologic processes, but uncertainty in the spatial heterogeneity of these linkages affects our ability to upscale or downscale the observed processes to other scales of interest: the bajada (large), the plot (medium), the point scale (fine). At the microsite scale (canopy or intercanopy), field investigations can yield point-scale information on particle size distribution and soil hydraulic properties, but relatively few have shown spatial patterns at the meter scale.

Obtaining hydraulic parameters in these heterogeneous soils is often tedious and field intensive. Soil texture or particle size distributions (PSD) are relatively easy to obtain, and have become common proxies for estimating hydraulic properties from pedo-transfer functions (PTF). These results can be obtained the laboratory using high-resolution laser-light scattering methods. Recent work by Meadows et al. (2006) at the Mojave Global Change Facility (MGCF) found spatial correlations for surface soil texture at approximately 50m for intercanopy locations. The resultant soil texture derived PTF resulted in highly variable Ks parameter field with a coefficient of variation of 0.47 and a spatial correlation length of 80m. The heterogeneity of the canopy/interspace distribution is likely to increase both the variability and spatial correlation of these hydraulic properties. The question that ultimately needs to be answered is whether wetting front variability in heterogeneous landscapes is generally dependent on the variability of flux at the upper boundary and less on the hydraulic property values.

We therefore seek to answer several questions that will directly tie into the SEPHAS project. From an ecohydrologic perspective, are highly heterogeneous properties influencing the soil-water balance of arid regions? From an experimental ecologist view, how vital is the spatial variability of hydraulic properties, either inferred or measured, to a random block experiment design, such as at the MGCF?

And how important is hydraulic property variability at the lysimeter scale, such as proposed by the SEPHAS project, where the soil-water balance can ultimately be determined?

The variability of hydraulic properties and its spatial relation to shrub and interspace will be determined using high-resolution measurements of Ks at 25-cm increments along transects radiating outward from a L. tridentata shrub to the interspace. Measurements will be made using a mini-tension infiltrometer array, which allows multiple point measurements to be conducted simultaneously and efficiently. Soil cores at each measurement site will be collected to obtain bulk density and soil texture from 0-10 cm depth. All resultant data will be explored for its spatial dependence and used to derive more site-specific hydraulic properties immediately surrounding the shrub site.

Tension infiltrometer array determining soil hydraulic parameters at 25-cm increments around a L. tridentata canopy

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