SOIL MOISTURE RESEARCH
Monitoring the amount of water in the soil and understanding the impacts of various soil moisture conditions are critical tasks that are being accomplished by meteorologists and researchers at the Oklahoma Climatology Survey. Citizens within the state of Oklahoma benefit greatly from the soil moisture knowledge disseminated from OCS - from farmers using data to aid in scheduling of crop watering to firemen being able to better pinpoint possible areas of danger for wildfires to forecasters enhancing thier computer models from the ingestion of soil moisture data. As a result of the efforts at OCS, Oklahoma has put itself in the forefront of public use and research of soil moisture with the Oklahoma Mesonet’s Soil Moisture Network.
The Oklahoma Mesonet is an automated network of 120 remote, meteorological stations across Oklahoma (Brock et al, 1995; Shafer et al, 2000; McPherson et al, 2007). Each station measures core parameters that include: air temperature and relative humidity at 1.5 m, wind speed and direction at 10 m, atmospheric pressure, downwelling solar radiation, rainfall, and bare and vegetated soil temperatures at 10 cm below ground level. In 1996, soil moisture sensors were installed at 60 Oklahoma Mesonet sites at depths of 5, 25, 60 and 75 cm (Illston et al, 2003; Illston et al, 2008). Based upon the initial success in using data from this original deployment, additional soil moisture sensors were installed at 43 additional Mesonet sites during 1998 and 1999 and at new stations since 1999. Currently, the Oklahoma Mesonet has soil moisture stations at 103 stations across the state and are shown in Figure 1.
A key aspect of the network of soil moisture sensors used by the Oklahoma Mesonet is that estimates of both soil-water potential and water content are collected every 30 minutes. The sensors measure soil moisture by taking the temperature of a wire, heating the sensor for a few seconds, pausing for a few seconds, and then taking the temperature a second time. Since heat dissipates more slowly in water (high heat capacity), wet soils will have a smaller change in temperature measured by the sensor than dry soils. This temperature difference allows hydrological variables such as soil water content, soil matric potential, and Fractional Water Index (FWI) to be calculated.
Soil water content is the physical amount of water per volume of soil. Different soil types (i.e. sand, clay, etc.) have different sizes of particles, which results in varying amounts of space available for water to fill. Soil matric potential is the force, like pressure, needed to move water vertically. When there is less water in the soil, a larger force is required to move the water. Soil matric potential is useful in agricultural situations, when it is important to know how effective certain plants are at removing water from the soil. However, soil matric potential is an exponential function, which makes it more difficult to implement in real time.
Since soil water content depends heavily upon soil texture, and soil matric potential is exponentially related to soil wetness, FWI was developed for an easy to use and understandable index. FWI tells us how far between the dry and wet extremes of the sensor a particular sensor reading resides. This unitless value ranges from very dry soil having a value of zero, to saturated soils having a value of one.
Figure 2: Typical 5cm Fractional Water Index in Early Summer.
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Figure 2 shows a typical image of soil moisture at a depth of 5 cm – about the length of your thumb – during the early part of the summer. The darker red areas show drier soils, while the blue areas indicate more moist soils. Maps like these, as well as other maps and time series graphs, are updated daily on the Oklahoma Mesonet website for public viewing.
This vast network of soil moisture sensors has led to many research opportunities, many of which would not have been possible without the Oklahoma Mesonet. The Oklahoma Climatological Survey has produced a detailed climatology of Oklahoma soil moisture. This climatology allows state water and agriculture officials to better understand what typical values of soil moisture should be, and when they are reaching extreme values for a particular time period. Illston et al. (2004) demonstrated that soil moisture values transition through four "seasons" throughout the year. Figure 3 shows an example of the moist plateau (I), transitional drying (II), enhanced drying (III), and recharge (IV) phases.
Additional studies have focused on the conditions of the droughts of 1998 and 2000 (Illston et al, 2003), which seriously impacted the state's economy through the lower production of agriculutral goods. A better understanding of how droughts occur and discovering signals to warn of impending droughts aid the advanced preparations for those who rely on soil moisture conditions for thier livelihood.
Air temperature and soil moisture anomalies can be seen in Figure 4 (A and B, respectively) with values of two widely used drought indicies shown in C and D of the same figure. As a result of the higher spatial resolution of the Oklahoma Mesonet compared to the climate zone resolution of the drought indicies, a clearer picture of the impacted areas of the state can be seen.
Further drought studies indicate fall and winter rainfall may help alleviate near-surface soil moisture problems during severe droughts, but deeper depths may continue to be impacted by the dry conditions (Illston et al, 2003; Illston et al, 2008).
Figure 5: Hollis, OK Soil Moisture Meteogram from 1998.
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Figure 5 shows that the drought can clearly be seen during the middle (summer) part of 1998 at Hollis, OK, but during the fall and winter, when rain has fallen, only the top two depths (red and yellow lines) recover from the drought conditions. This research demonstrated how overall drought impacts at a location can continue longer than just during and immediately following surface and near surface drought conditions.
There is still much to learn about soil moisture, and the Oklahoma Mesonet will continue to be an integral part of this process. More products will be made available in order to provide even more detailed soil moisture information to the public. Additional data sets will be produced for researchers to analyze. The discoveries which result will greatly benefit not only Oklahomans, but communities across the globe as well. Just as other portions of the Oklahoma Mesonet have pioneered how weather data is used, the soil moisture network will surely follow in its footsteps.
References
Brock, F.V., K.C. Crawford, R.L. Elliot, G.W. Cuperus, S.J. Stadler, H.L. Johnson, and M.D. Eilts, 1995: The Oklahoma Mesonet: A technical overview. J. Atmos. Oceanic Tech., 12, 5-19.
Illston, B.G. and J.B. Basara, 2003: Analysis of short term droughts in Oklahoma. EOS, Trans., AGU, Vol. 84, No. 17, p.157-161.
Illston, B.G., J.B. Basara, and K.C. Crawford, 2003: Soil moisture observations from the Oklahoma Mesonet. GEWEX News, Vol. 13, No. 3, p. 13-14.
Illston, B.G., J.B. Basara, and K.C. Crawford, 2004: Seasonal to interannual variations of soil moisture measured in Oklahoma. Intl. Journal of Clim. Vol. 24, No. 15, p.1883-1896.
Illston, B.G., J.B. Basara, D.K. Fischer, R.L. Elliott, C. Fiebrich, K.C. Crawford, K. Humes, and E. Hunt, 2008: Mesoscale Monitoring of Soil Moisture Across a Statewide Network. J. of Atmos. and Oceanic Tech, 25, 167-182.
McPherson, R. A., C. Fiebrich, K. C. Crawford, R. L. Elliott, J. R. Kilby, D. L. Grimsley, J. E. Martinez, J. B. Basara, B. G. Illston, D. A. Morris, K. A. Kloesel, S. J. Stadler, A. D. Melvin, A.J. Sutherland, and H. Shrivastava, 2007: Statewide monitoring of the mesoscale environment: A technical update on the Oklahoma Mesonet. J. Atmos. Oceanic Tech., 24, 301–321.
Shafer, M.A., C.A. Feibrich, D.S. Arndt, S.E. Fredrickson, and T.W. Hughes, 2000: Quality assurance procedures in the Oklahoma Mesonet. J. Atmos. Oceanic Tech., 17, 474-494.
