An evaluation of two inexpensive energy-balance techniques for measuring water use in flood-irrigated pecans (Carya illinoinensis)

Pecan (Carya illinoinensis) production in the southwestern US represents a significant proportion of both the irrigated farmland and the consumptive water use of crops. Increasing the irrigation efficiency of pecan orchards is essential to decreasing the amount of water applied while maintaining the productivity of the orchards. In order to increase the irrigation efficiency, it is necessary to measure water use for orchards of different sizes and ages. The most common system uses the sonic eddy covariance (SEC) technique, which utilizes a sonic anemometer and an open path water vapor sensor to measure orchard water use. This method is expensive and can be complicated to instrument, leaving a demand for cheaper and easier methods. The accuracy of two inexpensive micrometeorological systems (energy-balance techniques) were compared to an SEC system by measuring sensible heat flux (H) and latent heat flux (LE) densities over a mature pecan canopy in the Mesilla Valley of southern NM for two growing seasons. The energy-balance techniques evaluated in this study used one propeller eddy covariance (OPEC) and surface renewal analysis (SR) to measure H and then calculate LE as a residual of the energy balance. SR was evaluated at two measurement heights (z); canopy height (h) and the zero plane displacement height d = 0.7h. Both the SEC and OPEC systems were evaluated at z ∼ 1.3h. The SEC system measured H and LE using a three-axis sonic anemometer and a fast response, open path, infrared gas analyzer. When the daily total of H was positive, OPEC underestimated H by 13%. When the daily total of H was negative, OPEC overestimated H by 124%. For the daily LE evaluation, the OPEC underestimated LE within 8% for both years. The SR sensor underestimated daily LE by 8% at z = d and overestimated by 11% at z = h for both years. It is recommended that the OPEC system be used with z ≥ (d + 6) m and a correction factor of 1.13 when daily totals of H are greater than zero, otherwise a correction of 2.24 should be used. The SR system could be used at z = h using an alpha calibration of 0.5 or at z = d with an alpha of 1.1 to calculate LE in this tall, irrigated canopy. On a seasonal basis, the corrected OPEC estimated the water use within an error of 5% (0.06 m) of SEC for both study years, SR at z = d was within 14% (0.17 m) and SR at z = h was within 1% (0.01 m). Using the SR system to measure pecan consumptive water use was simpler to instrument than the OPEC but, it may need to be calibrated on-site by an SEC system negating its usefulness as a stand alone, water use measurement tool. The growing-season crop coefficients (K_c) for orchards like pecan can be measured by OPEC or SR reasonably. Measuring K_c by OPEC or SR for winter crops needs to be further tested because these systems overestimated pecan K_c in the dormancy seasons. SEC measurements had more ET and K_c outliers in the growing seasons than the OPEC and SR systems because SEC ET measurements are not constrained by energy balance. Using the OPEC or SR systems to measure pecan (or other tree crops) water use are accurate and cost-effective methods that can aid crop water and irrigation management.