A subsurface drip irrigation system for weighing lysimetry

Authors: Evett, Steven - Steve; Marek, Gary; Colaizzi, Paul; Ruthardt, Brice; Copeland, Karen

Submitted to: Applied Engineering in Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/14/2017
Publication Date: 2/1/2018
Citation: Evett, S.R., Marek, G.W., Colaizzi, P.D., Ruthardt, B.B., Copeland, K.S. 2018. A subsurface drip irrigation system for weighing lysimetry. Applied Engineering in Agriculture. 34(1):213-214. doi:10.13031/aea.12597.
DOI: https://doi.org/10.13031/aea.12597

Interpretive Summary: Conservation of scarce water supplies, increasing yields without increasing water use, and stabilizing yields as water supplies decrease are all important to farmers applying irrigation from the diminishing Ogallala Aquifer in the Southern High Plains. Although the predominant irrigation application method in the region is now center pivot sprinkler application, there is increasing use of subsurface drip irrigation (SDI), despite its higher cost. The increasing use of SDI is due to perceptions of improved yields with less water use. A research program was established at the USDA ARS Conservation & Production Research Laboratory, Bushland, Texas, to compare SDI and sprinkler irrigation systems. The study aimed to determine accurate values for crop yield, water use, losses of water to evaporation, and overall water productivity (yield per unit of water pumped – a key indicator of economic viability). The comparisons use the precision large weighing lysimeters at Bushland, which can measure crop water use with great accuracy. However, that accuracy can be diminished greatly during SDI water applications unless a way can be found to separate the water application rate from the crop water use rate. Scientists and engineers collaborated to devise and test an SDI system for the lysimeters that completely separated the increase of lysimeter weight due to irrigation from the decrease in weight due to crop water use. The new system operates successfully and allowed the scientists to detect large decreases in evaporative losses when using SDI, and significant increases in both yield and the yield per unit of water pumped for sorghum and corn crops. Other crops will be added as these studies continue to determine the economic payback from adoption of SDI.

Technical Abstract: Large, precision weighing lysimeters can have accuracies as good as 0.04 mm equivalent depth of water, adequate for hourly and even half-hourly determinations of evapotranspiration (ET) rate from crops. Such data are important for testing and improving simulation models of the complex interactions of surface water and energy balances, soil physics and plant biophysics that determine crop ET in response to rapid microclimate dynamics. When crops are irrigated with sprinkler systems or other rapid additions of water, the irrigation event is typically short enough that not much ET data are compromised by the lysimeter mass change due to irrigation. In contrast, subsurface drip irrigation (SDI) systems may take many hours to apply an irrigation, during which time the lysimeter mass change is affected by both ET rate and irrigation application rate. Given that irrigation application rate can be affected by pressure dynamics of the irrigation system, emitter clogging and water viscosity changes with temperature over several-hour periods, it can be difficult to impossible to separate the ET signal from the interference of the irrigation application. The inaccuracies in the data can be important, particularly for comparisons of sprinkler and SDI systems, since they are of the order of 8 to 10% of daily ET. We developed an SDI irrigation system to apply irrigations of up to 50 mm to large weighing lysimeters while limiting the period of lysimeter mass change due to irrigation delivery to approximately ten minutes by storing the water needed for irrigation in tanks suspended from the lysimeter weighing system. The system applied water at the same rate as the SDI system in the surrounding field, allowed irrigation over periods of any duration, but often exceeding 12 hours, without directly affecting lysimeter mass change and the accuracy of ET rate determinations, and allowed irrigation overnight without compromising lysimeter daily ET measurements. Errors in lysimeter ET measurements using the previous SDI system, which was directly connected to the field irrigation system, were up to 10% of daily ET compared with negligible error using the new system. Errors using the previous, directly connected, SDI system varied over time due to variable system pressure, and possibly due to water temperature (viscosity) changes and emitter clogging. With the new system, all of the water transferred to the lysimeter weighing system was eventually applied by the SDI system regardless of temperature, pressure or emitter clogging. Differences between planned and applied irrigation depth were less than 2% over the irrigation season.