ROSA GLADYS DELGADO DELGADO 3147501149

Gypsum sensors can be made easily by unskilled labour and can be very low-cost (~US$12 each).

In soils with good hydraulic conductivity (well structured loams and clays), where water can flow freely, sensors will equilibrate with a large volume of soil and be unaffected by small stones, cavities or plant roots adjacent to the sensor.

Resistance sensors can be automatically read and readings recorded (datalogging) using equipment dedicated to this use (Irrometer, M.K. Hansen and Measurement Engineering Australia companies); general purpose dataloggers with a capacity for AC resistance may also be used.

Gypsum sensors only work from the refill point to approximately six bars, much less than the wilting point suction for most plants. Changes in soil water tension in wetter or drier ranges produced no change in the resistance of the sensor.

In a sand or loamy soil, the conventional gypsum sensor is of limited value, as much of the soil water is gone before the fine pores in the gypsum begin to drain and the sensor registers a change, hence the limited utility of this device in its conventional form. The different porosity of the GMS sensor causes its useful range to be better adapted to sand or loam soils. The limited suction range of the conventional sensor is not such a problem in clay soils, particularly for crops that are not sensitive to mild stress. When a clay dries and reaches 150 kPa soil water tension (the point at which the sensor starts to change), the water content in most clays is still near the saturated water content. At the dry end of the gypsum sensor range (600 kPa), most clay will still deliver a large amount of water to a plant; and for many crops this range is ideal. For example, on clay soils the conventional gypsum sensor registers soil water tension in an ideal range for wine grapes, which are grown under controlled stress for fruit quality.

Neither kind of electrical resistance sensor can be reliably used to deduce soil water content. They are effective in determining the time to irrigate, but the decision as to how much to irrigate will depend on knowledge of the crop, soil and accumulated evapotranspiration. Gypsum sensors do not last indefinitely. Gypsum sensors rely on a continuing supply of calcium sulphate. As they wet and dry, the supply of calcium sulphate is leached from the sensor. Because the gypsum dissolves over time, the pore size distribution of gypsum blocks

changes over time, which causes the calibration to change. In neutral or alkaline soils the conventional sensor is expected to last around five years. In acid soils, however, the gypsum dissolves more quickly and the sensors may need to be replaced annually. Gypsum sensors cannot be recommended for soils with pH < 5. Need for replacement is usually obvious as the sensors remain ‘open circuit’ (large resistance) even in wet conditions. Caution is needed when using GMS sensors in mildly saline acid soils, as they may fail due to complete dissolution of the CaSO4 pellet without it being obvious.

Because the conductivity of ionic solutions is temperature sensitive, resistance sensors are temperature sensitive (as much as 20 kPa per 10°C, Shock, 2003), which is less problematic with deeper installation where soil temperature is more constant.

Like most other porous materials, the electrical resistance sensor is subject to hysteresis. This means that any given soil suction may correspond to several different soil water contents, depending on the prior water content history of the soil. In some applications this is a serious impediment to its use, but in irrigated agriculture and horticulture, this is not a critical factor because the irrigation process generally ensures that the sensor is returned to near saturation at the beginning of each irrigation cycle. Hysteresis can, however, present difficulties in soil water studies where wetting is incomplete, such as with some forms of subsurface drip irrigation.

9.3. CALIBRATION

Electrical resistance sensors can be calibrated using a pressure plate chamber, giving the drying curve of soil water potential vs. electrical resistance (Shock et al., 1998). Calibration should be done in the soil into which the sensors will be installed in the field. Using the field soil will elucidate some problems with soil–sensor contact and capillary barriers that may form if the pore size distribution of the sensor is quite different from that of the soil. Additional information from studies by R. Allen is available at http://www.kimberly.uidaho.edu/water/swm/, and information on the use of GMS in irrigation scheduling from C. Shock is available at http://www.cropinfo.net/granular.htm.

REFERENCES TO CHAPTER 9

Qualls, R.J., J.M. Scott, and W.B. DeOreo. 2001. Soil moisture sensors for urban landscape irrigation: effectiveness and reliability. J. Am. Water Resour. Assoc. June 2001. v. 37 (3) p. 547–559.

Shock, C.C. 2003. Soil water potential measurement by granular matrix sensors. Pp. 899–903 In B.A. Stewart and Terry A. Howell (editors). Encyclopedia of Water Science, Marcel Dekker, Inc. New York, NY.

Shock, C.C., E.B.G. Feibert, L.D. Saunders. 2002. Plant population and nitrogen fertilization for subsurface drip-irrigated onions. Special Report 1038. Oregon State University Agricultural Experiment Station. http://www.cropinfo.net/AnnualReports/ 2001/ondrip01.htm (accessed 31 March 2004).

Shock, C.C., K. Kimberling, A. Tschida, K. Nelson, L. Jensen, and C.A. Shock, 2003. Soil moisture based irrigation scheduling to improve crops and the environment. In Oregon State University Agricultural Experiment Station, Special Report 1038:227–234.

Shock, C.C., J. Barnum, and M. Seddigh. 1998. Calibration of Watermark soil moisture sensors for irrigation management. Irrigation Association. Proceedings of the International Irrigation Show pp. 139–146. San Diego, CA.

Taber, H.G., V. Lawson, B. Smith, and D. Shogren. 2002. Scheduling microirrigation with tensiometers or Watermarks. International Water Irrig. Vol. 22, No. 1. pp. 22, 23, 26.

CONTRIBUTORS TO DRAFTING AND REVIEW Main authors

Cepuder, P. Institute for Hydraulics and Rural Water Management, University of Natural Resources and Applied Life Sciences, Austria

Evett, S. USDA–ARS Bushland, USA

Heng, L.K. International Atomic Energy Agency Hignett, C. Soil Water Solutions, Australia

Laurent, J.P. Laboratoire d’Étude des Transferts en Hydrologie et Environnement, France Ruelle, P. Unité de Recherche “Irrigation”, CEMAGREF, France

Peer reviewers

Evett, S. USDA–ARS Bushland, USA

Heng, L.K. International Atomic Energy Agency Moutonnet, P. France

Nguyen, M.L. International Atomic Energy Agency

Overall coordinator