Analytical Methods:Moisture Monitoring

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Degradation processes linked to soil moisture
Soil water is essential for many biological, chemical and physical processes of degradation and therefore, is an important factor in the preservation of cultural remains. Many processes are inhibited by very wet or very dry soil conditions. Properties of soil water that may also be of importance are soil water potential, pH, conductivity and solute concentration.

Soil moisture content is particularly sensitive to seasonal and annual variation in temperature and rainfall (precipitation). Soil moisture levels can vary greatly from day to day; this variability can make it difficult to spot real trends in soil moisture without frequent repeat measurement.

Approaches to monitoring soil moisture
Soil moisture can be measured physically in the field either using an electronic probe, such as a theta probe, or by removing samples of soil for laboratory drying and analysis. This can be quite labour intensive because of the number and frequency of measurements that are typically required to build a picture of changing site conditions. Unless there are open sections or the site is to be repeatedly cored it can also be difficult to sample vertically through the soils and sediments. However, if physical samples of soil are required for other measurements, it is likely that the soils will need to be dried anyway and the measurement of soil moisture on these samples requires little extra work.

In-situ probe arrays can be installed above the watertable to monitor soil moisture at fixed intervals over a period of time. These may be linked to a datalogger otherwise site visits will need to be made to download data. To install an in-situ array over a series of depths may itself be destructive, but once installed further disturbance should be minimal. Repeat physical measurements over depth will need to involve repeated coring, or the maintenance of an open section, which is unlikely to be representative of conditions within the body of the site.

Sampling strategies
The sampling strategy for soil moisture will need to take account of the high degree of spatial and temporal variability in soil moisture content. soil moisture content is linked to changes from above (precipitation and surface evaporation) and below (groundwater) so a model of changes with depth is usually more useful than a map of surface soil moisture conditions. That being said it is important that spatially account is taken of soil, slope, geology and archaeological variation across the site.

The frequency of meaurement will depend in part on the reason for monitoring and the technique being used. In-situ arrays can collect a large quantity of data with short sampling intervals (hourly or daily) that can be used to build a picture of changing preservation conditions over the course of one or more years across a particular site. If a longer-term monitoring scheme is required, perhaps to monitor the effects of climate change or land management practices less frequent measurements may be more appropriate. At the very least an annual sampling scheme will need to be carried out at the same time of year to minimise seeasonal differences.

Soil moisture data can also be useful in correlation with dipwells and groundwater levels in order to model changing soil moisture and oxidation conditions with depth. Smit et al. (2006) recommend a minimum if one monitoring point per dipwell for such an approach.

Related techniques

 * Soil Moisture
 * Monitoring watertable levels

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