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Measuring Hydraulic Conductivity of Variably-Saturated Soils

at the Hectometer Scale Using Cosmic-Ray Neutrons

Item type text; Electronic Thesis

Authors Karczynski, Adam Michael

Publisher The University of Arizona.

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Downloaded 22-Jan-2017 17:59:59 Link to item http://hdl.handle.net/10150/323446



by Adam M. Karczynski ____________________________

A Thesis Submitted to the Faculty of the


In Partial Fulfillment of the Requirements For the Degree of



In the Graduate College



This thesis has been submitted in partial fulfillment of requirements for an advanced degree at the University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.

Brief quotations from this thesis are allowable without special permission, provided that an accurate acknowledgement of the source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his or her judgment the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author.

SIGNED: Adam M. Karczynski


This thesis has been approved on the date shown below:

28 May 2014 Marek Zreda Date Professor of Hydrology



1. Introduction…………………………………………………………….... 6

2. Field measurement using cosmic-ray neutron method……………....... 7

3. Laboratory measurements using HYPROP method………………...... 10

4. Comparison of the two methods………………………………………... 11

5. Advantages and disadvantages, problems and limits of applicability.. 12

6. Implications and outlook………………………………………………... 14

7. Appendix S1…………………………………………………………….... 20

8. Appendix S2…………………………………………………………….... 22

9. References………………………………………………………………... 24

–  –  –

1. Precipitation and soil moisture record……..………………………....... 16

2. Composite drying curve…………………………………………………. 17

3. Hydraulic conductivity curves and variance trends…………………… 18

4. Comparison of effective and aggregate hydraulic conductivity curves. 19 S1.1 Neutron count and soil moisture relationship……………………....... 20 S1.2 Environmental conditions………………………………………....…... 21 S2.1 Bucket model regression analysis……………………………………... 23 Measuring hydraulic conductivity of variably-saturated soils at the hectometer scale using cosmic-ray neutrons Adam Karczynski, Marek Zreda, and Trenton Franz* Department of Hydrology and Water Resources, University of Arizona, Tucson, Arizona, USA *Present address: School of Natural Resources, University of Nebraska-Lincoln, Lincoln, Nebraska, USA Corresponding author: M. Zreda (marek@hwr.arizona.edu) Abstract Hydraulic conductivity of variably-saturated soils is critical to understanding processes at the land surface. Yet measuring it over an area comparable to the resolution of land-surface models is fraught because of its strong spatial and temporal variations, which render point measurements nearly useless. We derived unsaturated hydraulic conductivity at the horizontal scale of hectometers and the vertical scale of decimeters by analyzing trends in soil moisture measured using the cosmic-ray neutron method. The resulting effective hydraulic conductivity remains close to its value at saturation over approximately half of the saturation range and then plummets. It agrees with the aggregate of 36 point measurements near saturation, but becomes progressively higher at lower water contents; the difference is potentially reconcilable by upscaling of point measurements. This study shows the feasibility of the cosmic-ray method, highlights the importance of measurement scale, and provides a route toward better understanding of landsurface processes.

1. Introduction Knowledge of field-scale hydraulic conductivity of soils is critical for understanding mass and energy transfer between the atmosphere and the land surface. Hydraulic conductivity values are necessary for land surface models to reconcile moisture distribution within a soil column [Lawrence et al., 2011], for models of water flow, infiltration, recharge, crop growth, and the transport of solutes, and for additional uses in the fields of ecology, agronomy, and engineering [Clemente et al., 1994; Smith et al., 1995; van Dam et al., 1997; Hillel, 1998; Reynolds et al., 2000; Pitman, 2003; Gutman and Small, 2005; Ghanbarian-Alavijeh, 2012]. But effective, or areaaverage, hydraulic conductivity (Ke) is difficult to measure, particularly in variably-saturated soils.

Direct methods rely on field or laboratory measurements [Dane and Topp, 2002; Schindler, 1980;

Peters and Durner, 2008] on small-scale samples, which is expensive and time-consuming.

Subsequent averaging of individual measurements and upscaling of the aggregate hydraulic conductivity (Ka) values to a field scale [Neuman, 1994; Zhu, 2004; Severino and Santini, 2005] introduces further uncertainty because the relationship between scales is poorly understood [Gutman and Small, 2005; Vereecken et al., 2007]. Indirect methods rely on assessing flow or states over an area and computing Ke using inverse modeling, which is fraught with difficulties associated with large-scale measurements of fluxes or states [Binley and Beven, 1989].

Here, we present a new method for determining Ke of soils at the scale of hectometers from observational data. It takes advantage of the recently-developed ability to measure soil moisture at the intermediate scale using cosmic-ray neutrons [Zreda et al., 2008, 2012; Desilets et al., 2010].

Our method (Section 2) involves measuring soil moisture using cosmic rays, creating a drying curve, evaluating fluxes out of the control volume, computing area-average infiltration rates, and equating these rates to Ke under the assumption of a unit gradient. The aggregate hydraulic conductivity (Ka) from 36 small samples collected within the control volume and measured in the laboratory using the simplified evaporation method [Schindler, 1980; Peters and Durner, 2008] (Section 3) were then compared to the Ke values from the drying curve evaluation and found different, but reconcilable by upscaling (Section 4).

2. Field measurement using cosmic-ray neutron method The field experiment was conducted within the Manitou Experimental Forest [Ortega et al., 2014], located 45 km northwest of Colorado Springs in central Colorado. Unsaturated hydraulic conductivity at the field scale was measured at the Manitou COSMOS (COsmic-ray Soil Moisture Observing System) site (39.1006° N, 105.1025° W) using changes in soil moisture derived from cosmic-ray neutrons between June and September 2010. The forest at the COSMOS site is of medium density (7 kg/m2 of dry biomass) and consists of mainly ponderosa pine. Two COSMOS probes were installed on a walk-up tower, ~1 m and ~20 m above the ground [Zreda et al., 2012;

see http://cosmos.hwr.arizona.edu]. During the study period, the average air temperature was ~15 °C and the total precipitation was ~200 mm [Asherin, 2011].

The cosmic-ray neutron probe measures moderated fast neutrons in air above the soil surface, where their intensity is inversely correlated with soil moisture [Zreda et al., 2008]. We used the raw moderated neutron count (Nr) shown in Figure 1 and applied corrections for temporal changes in pressure, atmospheric water vapor and incoming neutron intensity, and normalized the count rate (N) to the San Pedro site [Zreda et al., 2012; Rosolem et al., 2013].

At Manitou, the COSMOS probe measures soil moisture over a circular area with a diameter of ca. 800 m, computed using Desilets and Zreda [2013], and over a depth that varies from 15 cm when soil is near saturation to 30 cm when soil is drier (computed using Franz et al. [2012]). For the computation of soil hydraulic properties the horizontal footprint is the same 800 m, and the depth of measurement (ztotal) is set to 21 cm, based on the linear-regression analysis of soil moisture increases due to precipitation events (See Appendix S2 for details).

The mass balance for our control volume, per unit horizontal cross-sectional area, is:

–  –  –

where P is precipitation, ET is evapotranspiration in equivalent units (mm day-1), H is horizontal flow, I is infiltration, ztotal is measurement thickness, and d/dt is change in storage (m3 m-3 day-1).

Assuming negligible horizontal flow (H=0), limiting the evaluation to post precipitation drying curves (P=0), and subtracting ET estimated from limited eddy-covariance data [Francina Dominguez, University of Arizona, personal communication, April 2014] and supplemented with

meteorological data [Asherin, 2011], the equation becomes:

–  –  –

Infiltration can be described as the specific discharge in the Darcy-Buckingham equation for one dimensional flow. Under an assumed unit gradient [DiCarlo, 2003; Yeh, 1989; Hillel, 1976], K is

defined as a function of θ and Eq 2 can be written as:

–  –  –

Therefore, the hydraulic conductivity, which equals vertical flow rate at a specific soil moisture content, can be computed directly from drying curves generated from COSMOS data (Figure 1).

Ten individual drying curves, each following a rain event, were combined to reduce the noise in the data and to expand the range of θ values available for evaluation. This process is applicable as drying curves for different precipitation events are similar (i.e., they plot on top of each other). The individual drying curves were collated by first identifying the peak values for θ from the COSMOS data set and meteorological data from Asherin [2011], and following each event’s drying curve.

The maximum peak θ, on August 4, was then identified and the time of this peak was set to zero.

This curve is the master drying curve. The peak θ for the remaining drying curves were then matched to an average θ value on the master drying curve. The combination of all events yielded the composite drying curve (Figure 2).

Once the composite drying curve was completed, the θ values at each hour increment were averaged to reduce potential bias from the θ matching procedure, and a three-parameter exponential function was fitted to the data (Figure 2). The derivative of the exponential function was then used to determine daily rates of change in volumetric water content. By removing estimated evapotranspiration values, and applying Eq 3, we can directly compare our measured values to aggregate values from the samples measured in the laboratory (next section).

Near saturation, the Ke value is ~1.0 cm day-1. As θ decreases, the values of Ke decrease slowly until θ nears 0.20 m3 m-3 at which point the change in Ke accelerates until θ falls below 0.12 m3 mwhere it drops sharply, marking the cessation of downward flow. This drastic change in Ke when θ is less than 0.12 m3 m-3 corresponds to the mean field capacity of the samples.

3. Laboratory measurements using HYPROP method The HYdraulic PROPerty analyzer (HYPROP) measures pressure at depths of 12.5 mm and 37.5 mm within a soil column and mass changes to generate hydraulic conductivity and water retention functions via the evaporation technique [Schindler, 1980; Schindler and Müller, 2006; Schindler et al., 2010]. Measurements obtained using this technique have been compared to standard methods in numerous studies and the HYPROP method is considered reliable [Peters and Durner, 2006; Peters and Durner, 2008; Schindler et al., 2012; Öztürk et al., 2013].

Thirty-six undisturbed soil samples were collected within the COSMOS footprint in August 2013.

Each sample was collected by digging to a desired depth (2.5 cm, 12.5 cm, and 22.5 cm), driving a HYPROP sampling ring (80 mm diameter, 50 mm length) into the soil, excavating the ring, and securing the sample with plastic caps and tape to protect it during transport. In the laboratory, each sample was saturated from below over a period of three days and then secured to a HYPROP device and placed on an analytical balance. Pressure (tension) and mass were recorded every minute for the first sixty minutes and hourly thereafter for a period of seven to ten days. When the mass change became negligible, the sample was removed from the HYPROP device, dried in an oven at 105 °C for 48 hours, and weighed to determine the dry bulk density and porosity.

Hydraulic conductivity values from HYPROP measurements were corrected for temperature differences between the laboratory experiments (~24°C) and the field (~15°C), and plotted as functions of θ. The arithmetic, harmonic, and geometric averages of the sample conductivities were then determined from the 36 individual curves (Fig. 3a).

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