1. Site Selection and Installation of Soil Moisture Sensors at Oregon & Washington SNOTEL Sites By: Melissa Webb and Sheila Strachan Oregon & Washington NRCS Snow Surveys Site Selection for Soil Monitoring Installation of Soil Sensors Presentation and Delivery of Soil Moisture Data SENSOR INSTALLATION (continued) After the 40" sensor is in place, backfill the auger hole with the material that was removed and tamp the backfill into place. When packing the soil around the sensor, be careful that you don't pull or loosen the sensor. Carefully run the wires away from the sensor as the orientation of the excavation allows. It is important to avoid creating a drip line for water to follow along the wire straight to the sensor. Form a drip loop with the extra wire. Next, carefully backfill the soil excavated from the 22-25" section. Tamp into place as you backfill. Install the 20" sensor horizontally into the pit face. The center tong should be at the 20" depth. Create a drip loop with the wire, group it with the wire from the 40" sensor and begin to backfill the 22” and higher material. Backfill up to the 10" depth, tamping as you go. Install the 8“, 4“, and 2" sensors along the pit face in a staggered pattern, so that they are not vertically aligned. Carefully backfill the soil in the rest of the pit and leave drip loops in all the wires. Make sure the surface soil horizon covers the 2" sensor very well, because some settling may occur in the pit which could expose the 2" sensor if it is not covered adequately. Gather all the wires together at the surface and seal the end of the conduit with duct seal putty. When all the sensors are in place and the installation is complete, bury the conduit in the trench. DATA COLLECTION Soil moisture and temperature data at Oregon and Washington SNOTEL sites is collected every 6 hours using a Campbell Scientific CR10X data logger and a customized MUX board (Figure 7). The data is then transmitted via meteorburst radio to the central SNOTEL database in Portland, Oregon. The data is available on the NRCS Snow Survey webpage in near-realtime. Figure 7. Data collection and transmission equipment for soil monitoring at a SNOTEL site. Data collected from the SNOTEL soil monitoring network is available near-real time on the Oregon NRCS Snow Survey webpage. Soil moisture and temperature data are provided in both tabular and graphical format for all installed sensors at each site (Figures 9, 10, and 11). Two graphs are updated monthly – one depicts all soil moisture data since sensor installation (Figure 10) and one depicts soil moisture data for the current calendar year (Figure 11). In addition to soils data, the snow water and precipitation traces are also included on these graphs to help users understand the hydrologic processes driving soil moisture data. These graphs are very helpful in accessing historic trends at each site and current conditions in relation to these trends. In addition to soil moisture data, the web page for each soil monitoring site also includes photos of the soil profile and vegetation community as well as links to the site-specific soil pedon description, soil series information and lab characterization data. (Figure 8). Figure 11. Graph of soil moisture data for the current calendar year. Data is updated monthly. Figure 1. Map of the 29 soil monitoring sites co-located with SNOTEL sites in Oregon and Washington Figure 10. Graph of soil moisture data collected since sensor installation, updated monthly. Figure 8. Webpage access to soils data. Individual site pages contain metadata as well as soils & SNOTEL data. Figure 9. Tabular presentation of soil moisture and temperature data. Figure 2. Sourdough SNOTEL soil monitoring installation. Above-ground evidence suggests little or no past major soil disturbance. Figure 3. Skookum SNOTEL site – located on an old log landing. Multiple visual clues are present to suggest past soil disturbance, which makes this site a poor candidate for soil monitoring. Objectives for Soil Monitoring at SNOTEL Sites Soil monitoring is part of a new generation of weather stations designed to capture and record climate variables. Since inception is the late 1970’s, the role of the SNOTEL network has evolved in response to the increasing demands of water users. SNOTEL data is critical to the production of water supply forecasts, while also fulfilling many other requirements for hydrological and climatological data useful in natural resources management and research. Under the guidance of the National Water and Climate Center (NWCC), SNOTEL is being managed as a dynamic climate service network that provides data and analysis required for integrated ecosystem management. Water supply forecasts produced by NRCS hydrologists have historically relied on proxies for soil moisture conditions. With the growing network of reliable soil monitoring stations, forecasters will likely be able to incorporate actual soil moisture observations into forecasting procedures in the near future. In addition, soil scientists use the data to confirm soil moisture and temperature regimes previously estimated from plant associations. Finally, soil moisture data collected at SNOTEL sites is available from the NRCS webpage for unlimited applications and research by scientists and resource managers. Since the late 1990s, many SNOTEL sites across the western US have been instrumented with soil moisture sensors. There are currently 258 SNOTEL sites with soil monitoring. Figure 1 shows the 29 soil monitoring sites located in Oregon and Washington. There are several important factors affecting the location a soil monitoring site. Currently, soil sensors in the SNOTEL soil monitoring network must be installed within 100 feet of a SNOTEL site, due to cable length limitations. New technologies for expanding the range of sensor installations are in testing. In addition to these location limitations, the following factors should be taken into consideration when deciding where to locate a soil monitoring site: SOIL DISTURBANCE Soil disturbance can significantly alter the soil properties related to soil moisture retention and infiltration. Disturbance can also alter the properties of the soil that help define the soil type. Because of this, soil sensors should be installed on sites with minimal soil disturbance. Some examples of soil disturbance are: Soil compaction Loss of surface organic matter Mixing of soil strata Surface erosion (wind, water) Fire While most of these types of soil disturbance create evidence on the above-ground landscape, it is critical to keep in mind that there can be historic disturbance to the soil that may be hard to detect when only looking at above-ground evidence. Most types of soil disturbance take a very long time before their effects are no longer present in the soil. The above-ground landscape may return to a pre-disturbance state long before the soil does. It is important to know the history of land use on a proposed soil monitoring site. Common causes of soil disturbance due to historic land uses are heavy equipment used for logging or road building, wildfire, construction, and piling of woody debris or other organic material. Figure 2 shows a SNOTEL site that has no visual clues for past soil compaction. Historic land use was evaluated to be free of past ground disturbance, so this site was chosen as a soil monitoring site. In contrast, Figure 3 shows an example of a landscape that has many visual clues to previous soil disturbance – piles of woody debris, compaction, excavated level area used for log-landing and even-aged trees surrounding the site. This SNOTEL site would not be a good candidate for soil monitoring. SOIL TYPE When using soil moisture data to help predict water supply in the basin, it is preferable to instrument a soil type that is well represented within the region. Because the soil moisture data that is collected at SNOTEL sites will be extrapolated across the watershed, a soil that is common in the basin will have a stronger relationship with the measurements at the sensor level. Thus, if the soil holding the sensors has similar moisture-holding characteristics to many other acres of that soil in the watershed, the sensor data will be much more meaningful at the larger scale. If possible, select a location that: Has a common soil type (using a regional soil map) Is part of a common landform (not in a micro-habitat that is of small extent in the surrounding landscape) Has a common vegetation community (which is good guide to the underlying soil type) Figure 4 shows the spatial distribution of soil types in the region around Annie Springs SNOTEL. The soil series at the site is Castlecrest, which is represented in 47% of the surrounding area. This is a very high representation of soil type. The soil type at many sites will not be as well-represented, but knowledge of the soil type representation can help avoid installing soil sensors in a soil type that is poorly represented in the watershed. Installation procedures for soil monitoring at SNOTEL sites were created in cooperation with the NWCC and the National Soil Survey Laboratory. The soils sensor used is the Hydra Probe from Stevens Water Monitoring Systems. The standard for NRCS soil monitoring sites is to install 5 sensors at the following depths: 2”, 4”, 8”, 20”, and 40” below mineral soil surface. If soils are less than 40” or are too rocky for sensor installation at all depths, the last sensor is installed above bedrock contact (or deepest practical depth) and the sensor depth is noted. The soils at each site are sampled and described by a soil scientist. The soil scientist excavates a soil pit to fully describe the soil and takes bulk samples for lab analysis. Bulk density clods are also removed from this pit. A second, smaller soil pit is used for the sensor installation. This pit is located away from the sample pit to avoid disturbance, but close enough to ensure that the soils described and instrumented will be similar. SITE PREPARATION Excavate a hole no larger than 25” x 25” square and 25” deep for the sensor installation pit. It is very important that the soil sensor excavation is disturbed as little as possible. As soil layers are removed from the sensor excavation, place them in separate piles in order on a tarp (Figure 5). To best re-create the original soil horizons, these soil layers should be replaced in the pit in the same order they were removed. Trench from the location of the power source and data logger to the sensor installation pit. Assemble rigid or flexible PVC conduit to protect the sensor wires. Check that there is enough cable length to reach up through the soil pit and through the conduit to the data logger. Label sensor wires with sensor depth or position at both ends – the sensor end and the end that will be hooked up to the data logger. Thread the sensor wires through the conduit, leaving long cable lengths on either end. SENSOR INSTALLATION Before installing sensors into the soil, connect the wires to the datalogger and power source. Test each sensor separately in moist soil to make sure that it is working as expected. A small cup with moistened soil works well for testing because each sensor should give very close to the same reading for soil moisture and temperature. Install the 40" sensor first. Use an auger to dig from 25” to 40“ (or as deep as possible if the soils are less than 40”) (Figure 6). Insert the 40" sensor vertically into the undisturbed soil at the bottom of the auger hole. The sensor tines should be firmly in contact with the soil along their entire length. A customized 1” PVC pipe can be used to "spear" the sensor and push it down into the soil. This method can be easier than inserting the sensor manually down an auger hole that is only slightly larger than a hand. Figure #. Installation of Hydra Probes at 2”, 4” and 8” in the soil profile. Figure 6. Use of a soil auger to install the 40” sensor with minimal soil disturbance Figure 5. Soil pit excavation for soil sensor installation. Note that the soil horizons were removed carefully for replacement in the pit after sensor installation. Figure 4. Map of soil type distribution in the region surrounding Annie Springs SNOTEL site and the legend showing the spatial extent of each soil type. The soil type present at the soil monitoring site is highlighted in yellow on the map to show the representation of the sampled soil in the surrounding landscape. This data and map were generated using the Web Soil Survey (http://websoilsurvey.nrcs.usda.gov/), which is provided by NRCS and National Cooperative Soil Survey. Oregon & Washington Soils Data is available at http://www.or.nrcs.usda.gov/snow/