Evaluating Surface-Soil Water Content by Measuring Reflectance 1

Water’s property to absorb certain wavelengths in the near infrared was the basis for developing a reflectometer to measure reflectance of near-infrared radiation from a soil surface. The reflectometer’s essential elements include: source of infrared radiation, optical system, integrating sphere, detector, light chopper, amplifier, and meter system. The radiation from an incandescent lamp was filtered with narrow-band pass filter, chopped and allowed to strike the test surface, where it was either absorbed or reflected onto the surface of the integrating sphere. The intensity of the reflected radiation was measured with a lead sulfide detector and appropriate amplifier and meter. The reflectance as a function of water content was measured for filter paper and several soils at 1.30, 1.45, 1.65, and 1.95 pm. Although at low water contents soil properties (other than water content ) strongly influenced soil reflectance, at 1.95-pm wavelength-the most prominent absorption band of liquid waterthe reflectance-water content relationship tended to be log linear. Additional lndex Words: reflectometer, soil erosion, integrating sphere, light chopper, infrared detectors. ENERALLY, agriculturalists evaluate soil water to deG termine how its content and activity change in time and space in the soil’s root zone volume. Of the many methods used, none accurately indicates water status at the most transient position, the surface, where water content may change rapidly because of precipitation, irrigation, or evaporation. Even though water status of the thin surface layer may be inconsequential when considering totalvolume change of water in the entire soil volume or the number of roots occupying the surface volume of soil, it should be considered carefully. Its assessment is required in many soil-plant atmosphere simulation models; furthermore, the water status greatly influences a soil’s wind erodibility. Chepil (6) showed that the resistance of the uppermost soil particles against wind erosion was, on the average, proportional to the equivalent moisture squared, when equivalent moisture was defined as the ratio of the amount of water held in the soil to the amount held by the same soil a t 15 atm percentage. This was the basis for developing a climatic factor (7) in the wind erosion equation (24) used to predict the amount of soil that will erode from a given agricultural field and to determine the conditions necessary to reduce potential erosion to a tolerable amount. Surface soil water must also be assessed to determine the time required for the soil to dry to wind-erodible dryness for various climatic and soil conditions. Contribution from the Agricultural Research Service, USDA, in cooperation with the Kansas Agr. Exp. Sta. , Manhattan, Kan. Dept. of Agronomy Contribution No. 1408. Received 30 May 1974. Approved 5 Nov. 1974. Soil Scientist and Agricultural Engineer, respectively, North Central Region, ARS, USDA, and Electronics Technician, Evapotranspiration Lab. , Kansas State University, Manhattan, Kan. 66506. Water’s property to absorb certain wavelengths in the near-infrared electromagnetic spectrum suggests that an instrument might be developed to measure surface soil water. Curcio and Petty (10) found five prominent absorption bands of liquid water in the spectral range 0.70 to 2.50 pm. Absorption coefficient was the highest for 1.94-pm wavelength, which has been used in absorption techniques to measure moisture in liquids, solids, and methanol extracts of seeds (13, 19, 20). Although such a technique would not be feasible for soil in situ, one minus absorption or reflection (assuming no transmission) apparently would be feasible (2, 3, 4, 22). Bowers and Hanks (3) demonstrated the potential of using reflectance measurements to determine soil water content by showing that a 1.9-pm wavelength, for a Newtonia silt loam, reflectance decreased as water increased. Since we were convinced that a method to evaluate soil water at the surface was needed and knew that water can absorb certain radiation wavelengths, we developed and evaluated an instrument to indicate water content at the soil atmosphere interface by measuring reflectance. THE INSTRUMENT DESCRIBED A device to measure surface soil water content by infrared reflectance has these essential elements: source of infrared radiation, optical system, light chopper, integrating sphere, detector, and amplifier meter system. These elements, their relationship to each other, and radiation path from source to test surface are shown in Fig. 1. Figure 2 is a photograph of soil reflectometer and readout instrument. Radiation Sources-Many infrared sources consist of tungsten filament incandescent lamps that operate at temperatures of 2,400 to 2,900 K (2, 4, 5, 20). We used a tungsten filament lamp rated at 6.3 V but operated at 12.0 V. Just below the radiation source is an iris diaphragm used to control the diameter of the radiation beam at the sample port. Optical System-The optical system (with associated lenses and filters) is used to transmit the desired radiation band from source to test surface, where a portion of the radiation is reflected back into an integrating sphere and onto the IR detector. The holder beneath the diaphragm contains three compartments for 2.5-cm diameter filters; thus, simply by repositioning the holder, a different filter can be placed in the path of the radiation beam. Figure 3 shows transmission characteristics of the filters we used; each produced a different wavelength band. The 1.95-pm and 1.45-pm bands coincided with absorption bands of liquid water. The filtered radiation passed through two equiconvex lenses, mounted so that the lamp filament was at one focal point and the light chopper tines at the other. The chopped radiation passed through a small port on the top of the integrating sphere immediately below the light chopper, then to the sample port. Light Chupper-Using interrupted radiation with AC amplification was considered an important advance (12, 16). Until recently, a synchronously driven rotating sector was used (17, 18) to produce an alternating signal from the detector. We now use tuning fork choppers (11, 15) which offer many advantages over the synchronous motor and rotating sector. We chopped the radiation beam with a Bulova3 tuning fork light chopper at a frequency and tine amplitude of 400 Hz and 2.5 mm, respectively. Although 400 Hz is less thm the optimum frequency, it