Infiltration rates can be measured at a point using a variety of methods described here, each appropriate for certain conditions. However, because of the large temporal and spatial variability of infiltration processes, catchment average infiltration rates may be desired, which can be obtained through the water balance analysis of rainfall-runoff observations.31
Ring Infiltrometer. This simple method is most appropriate for flood irrigation or pond seepage infiltration. A cylindrical metal ring is sealed at the surface and flooded. Intake measurements are recorded until steady-state conditions are reached.32 If the effects of lateral flow are significant, then a double-ring infiltrometer can be used. Due to ponding conditions within the ring, observed infiltration rates are often higher than under natural conditions.33
Sprinkler Infiltrometer. This method is appropriate for quantifying infiltration from rainfall. Artificial rainfall simulators are used to deliver a specified rainfall rate to a well-defined plot. Runoff from the plot is measured, allowing computation of the infiltration rate.34'35
Tension Infiltrometer. The tension or disk infiltrometer employs a soil contact plate and a water column that is used to control the matric potential of the infiltrating water. By varying the tension, the effect of different size macropores can be determined.36'37
Furrow Infiltrometer. This method is useful if information on infiltration of flowing water in irrigation furrows is desired. Either the water added to a small section of blocked off furrow to maintain a constant depth or the inflow-outflow of a furrow segment can be monitored to determine the infiltration characteristics of the system.38
502 INFILTRATION AND SOIL MOISTURE PROCESSES 5 SOIL MOISTURE MEASUREMENT
Soil water content can be determined directly using gravimetric techniques or indirectly by inferring it from a property of the soil.39'40
Gravimetric. The oven-drying soil moisture measurement technique is the standard for calibration of all other methods but is time consuming and destructive. The method involves obtaining a wet soil sample weight, drying the sample at 105°C for 24 h, then obtaining the dry sample weight [see Eq. (2)].
Neutron Thermalization. High-energy neutrons are emitted by a radioactive source into the soil and are preferentially slowed by hydrogen atoms. The number of slow neutrons returning to the detector are a measure of soil moisture.
Gamma Attenuation. The attenuation in soil of gamma rays emitted from caesium-137 is directly related to soil density. If the soil's bulk density is assumed to be constant, then changes in attenuation reflect changes in soil moisture.41
Time-Domain Refiectometry (TDR). TDR measures the soil's dielectric constant, which is directly related to soil moisture, by measuring the transmit time of a voltage pulse applied to a soil probe.
Tensiometric Techniques. This method measures the capillary or moisture potential through a liquid-filled porous cup connected to a vacuum gage. Conversion to soil moisture requires knowledge of the water retention characteristic.
Resistance. The electrical resistance or conductivity of a porous block (nylon, fiberglass, or gypsum) imbedded in the soil depends primarily on the water content of the block. However, because of salinity and temperature sensitivity, measurements of these sensors are of limited accuracy.42
Heat Dissipation. Changes in the thermal conductivity of a porous block imbedded in the soil depend primarily on the water content of the block. The dissipation of a heat pulse applied to the block can be monitored using thermistors, then the soil water content can be determined from calibration information.
Remote Sensing. Soil moisture can be remotely sensed with just about any frequency where there is little atmospheric absorption.43 But, it is generally accepted that long wavelength, passive microwave sensors have the best chance of obtaining soil moisture measurements that contain little error introduced by vegetation and roughness and offer great potential to remotely sense soil moisture content with depth due to differential microwave absorption with varying dielectric constant.44
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