|Evett, Steven - Steve|
Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/27/2009
Publication Date: 8/20/2009
Citation: Tolk, J.A., Evett, S.R. 2009. Lysimetry versus neutron moisture meter for evapotranspiration determination in four soils. Soil Science Society of America Journal. 73(5):1693-1698.
Interpretive Summary: Measurement of the water loss from plant and soils is important for the management of our fresh water resources. Many methods exist to measure water loss, and we compared the results from two. One method measures water loss as a change in weight of a block of soil in a container from one reading to the next. This method is considered to be very accurate, but it is expensive to build and operate the equipment. The other method determines the water content of the soil by using an instrument to indirectly determine the number of water molecules in the block of soil. In this method, water loss is estimated as the difference between the numbers of water molecules from one reading to the next. This method is less expensive and can be easily installed in many locations. Both methods require additional information about water gains such as precipitation and irrigation and water losses such as drainage out of the block of soil. When the instrument from the second method is correctly adjusted to the soil’s water holding abilities, and water gains and losses are known, the second method can provide the information about water loss almost as accurately as water loss measured by weighing the soil.
Technical Abstract: Knowledge of evapotranspiration (ET) is vital for the management of our fresh water resources. One method for determining ET is through the measurement of the soil water balance where ET is the residual calculated from the change in soil water storage plus rainfall and irrigation and minus drainage and runoff. The objective of this research was to compare the ET calculations where the change in soil water storage was measured using the neutron moisture meter (NMM) versus using weighing lysimeters. Cotton (Gossypium hirsutum L.) was grown in 2006 and 2007 in weighing lysimeters with a 0.75-m**2 surface area and 2.3-m depth containing monolithic cores of four soils, centrally located NMM access tubes, and drainage systems. The texture of the four soils ranged from fine sand to clay loam. The NMM was field-calibrated at the core collection sites. The change in soil water storage was measured daily with the weighing lysimeters, and at intervals ranging from 12 to 60 days using the NMM. Drainage and rainfall were also measured. The ET calculated using the NMM method (ETnmm) tended to be within 3% of the ET measured using the lysimeter (ETlys) for the clay loam and sandy loam soils, but ETnmm was less than ETlys by about 8% in the fine sand, probably due to errors created by the timing of the drainage and NMM measurements. When all other soil water balance components are accurately quantified, a field-calibrated NMM can accurately determine changes in soil water balance for the calculation of ET.