|Xiao, Xinhua -|
|Heitman, Joshua -|
|Ren, Tusheng -|
|Horton, Robert -|
Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 7, 2013
Publication Date: N/A
Interpretive Summary: Most of the water that falls as rain or snow in temperate climates evaporates back to the atmosphere. Some of the water evaporates from the soil surface and some from plant leaves. Usually, growers would prefer that the amount of evaporation from the soil be small to keep most of the water available for plants. Methods to measure evaporation from soil beneath plants can be difficult and not very accurate. A new method was tested that uses very shallow and accurate measurements of heat transfer near the soil surface to estimate evaporation. Measurements were made beneath a corn canopy in Iowa. Three sensors were placed in the soil, one in the plant row, in between the plant row, and one between the plant row with no roots present. These measurements were compared with another method where large soil cores are weighed to determine the water lost to evaporation. The new method agreed well with the former method but has the advantages of allowing continuous measurement and requiring much less soil disturbance. This research is of interest to scientists and land managers interested in improving measurements of evaporation and water use efficiency.
Technical Abstract: Soil water evaporation is an important component of the water budget in a cropped field. Few methods are available for continuous and independent measurement of soil water evaporation. A sensible heat balance (SHB) approach has recently been demonstrated for continuously determining soil water evaporation in the laboratory and for bare surface field conditions. The applicability of SHB evaporation measurements beneath crop canopy cover has not been evaluated. In this study, we tested the SHB approach using heat-pulse (HP) sensors to estimate evaporation occurring beneath a full maize (Zea mays L.) canopy. We also implemented a modified SHB approach incorporating below canopy net radiation, which extended the range of conditions over which SHB is applicable. Evaporation was measured at three positions; row (R), interrow (I), and interrow with roots excluded (IE). Evaporation rates were generally small, averaging < 0.7 mm d-1 across all measurement dates and positions, and measurement methods during the drying period. SHB evaporation estimates varied between positions R, I, and IE, with cumulative totals of 4.4, 7.4, and 7.9 mm, respectively, during a 12 d drying period. Lower soil water contents from plant water uptake reduced evaporation rates at position R more appreciably with time after rainfall compared to other measurement positions. Positions I and IE provided similar evaporation patterns. Evaporation estimates from SHB at positions R and I were compared to microlysimeter evaporation measurements on eight days. Correlation between approaches was modest (r2 = 0.61) but significant (p < 0.001) when compared separately at R and I positions. Correlation improved (r2 = 0.81) when evaporation estimates were combined across positions, with differences between SHB and microlysimeter evaporation estimates typically within the range of values obtained from microlysimeter replicates. Overall, results suggest a good potential for using the SHB and modified SHB approaches to determine soil water evaporation in a cropped field. The SHB approach allowed continuous daily estimates of evaporation, separate from evapotranspiration and without destructive sampling.