B-1
Appendix B. How Many Soil Water
Content Measurements Are Enough
B.1 Introduction to Spatial Variability
Soil water content can vary significantly among several locations which are
near each other and apparently similar. Water content measurements using the
most accurate methods available provide evidence of significant differences in
soil structure and texture even when the measurements are limited to an area of
only 1 square meter. The degree of variability is dependent on many factors
including presence of coarse fragments (rocks), micro and macro-fauna activity,
erosion, management practice, and plant root activity. The difference in soil
physical properties from location to location and the subsequent difference in
soil water characteristics is commonly referred to as spatial variability of soil
hydraulic properties. Any characterization of an area that will be used to
manage or inventory water resources in that area must account for this
variability. It would not be prudent to schedule irrigation of a cropped field
based on a single measurement. Basic statistical methods are easily applied to
define how many measurements are needed to provide a specific level of
confidence that the area of interest has been adequately characterized. This
section of the appendix will demonstrate this statistical approach.
B.2 An Example
As an example, the results of 12 volumetric water content measurements taken
within a 2 meter radius on a well established and level lawn are presented.
(The measurements were not taken with a HydroSense.)
TABLE B.2-1. Sample Water Content Data
10.7
7.4
12.7
12.5
mean
11.9
11.9
12.5
12.1
14.0
stdev
1.74
13.4
12.5
10.2
12.5
range
6.6
B.3 Some Background on Confidence Intervals
Statistical approaches are based on probability theory. A confidence interval is
a different way to express probability. Consider measuring the water content at
many different locations in a volume of soil. The measurements will differ in
value but will be distributed about a mean value. If another measurement is
then taken, the probability can be calculated that this measurement will fall
within a specified interval which lies below and above the mean value.
Conversely, given a probability value, a water content range can be calculated
into which the measured value is likely to fall. The sum of the amounts below
and above the mean value is referred to as the confidence interval. The
confidence interval becomes smaller as the probability increases.
Summary of Contents for Hydrosense
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