Design and Construction of a Precision Weighing Lysimeter in Southeast Colorado

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Abstract. Accurate estimates of crop evapotranspiration (ET) are needed to effectively manage irrigation resources in the Arkansas River basin in Colorado and to maintain compliance with the Arkansas River compact with Kansas. This was a major impetus for the construction of a precision weighing lysimeter in the Arkansas River basin at the Colorado State University (CSU) Arkansas Valley Research Center (AVRC) near Rocky Ford, Colorado. The objective of this article is to describe the design and construction of the weighing lysimeter and characterize its performance and unique features. The main components of the lysimeter facility are the foundation, the scale system, the soil monolith tank, and the outer tank that houses the aforementioned components. The foundation, which was 4.12 m below the ground surface, consisted of a reinforced concrete slab 2.00 m wide by 6.31 m long and 0.20 m thick that was anchored to six square shaft helical anchors. The outer tank was secured onto the foundation and had a rectangular floor area of 6.10 m × 1.79 m (10.92 m2), an interior vertical clearance of 2.15 m, and walls made of reinforced 8 mm thick steel plates. The floor scale system (mechanical levers and load cell) was installed inside the outer tank and had a gross capacity of 17 Mg. The monolith tank (1.50 m × 1.50 m area, 2.44 m depth, 10 mm steel walls) containing an undisturbed soil profile was set on the scale system. The lysimeter facility was installed in the middle of a 3.5 ha field. Calibration of the scale system resulted in a linear response (R2 = 1.000), with an equivalent conversion coefficient (slope) of 151.09 mm H2O (mV V-1)-1. The sensitivity of the scale system was 0.023 mm of water, which is sufficient for measuring diurnal (15 min to hourly) changes in ET and soil water. Load cell readings taken at a frequency of 0.5 Hz were averaged in 15 min intervals (450 readings per 15 min) to filter out the measurement noise that was attributed to wind. The lysimeter was found to adequately detect ET, irrigation, and precipitation perturbations with an actively growing alfalfa hay crop ( L.) in 2011. The lysimeter facility is a state-of-the-art tool for quantifying ET of irrigated crops in the lower Arkansas basin in southeast Colorado. Keywords: Calibration, Evapotranspiration, Load cell, Weighing lysimeter.