Bowen Ratio Energy Balance System-Measured Evapotranspiration, Surface Energy Fluxes, Single and Basal Crop Coefficients and Energy Partitioning for Subsurface Drip-Irrigated Maize Canopy

Highlights Crop evapotranspiration (ETc), single and basal crop coefficients (Kc and Kcb), transpiration (T), and evaporation (E) were quantified for subsurface drip-irrigated maize. Bowen Ratio Energy Balance System (BREBS)-measured seasonal ETc was 667 mm in 2005 and 571 mm in 2006. BREBS energy balance closure errors were very low (2.6 W/m2 in 2005 and 3.6 W/m2 in 2006) with 97%-98% closure. Magnitude and timing of occurrence of maximum Kc and Kcb exhibited interannual variation. FAO56 method overestimated BREBS-measured ETc by 9.3% (root mean squared error, RMSD = 1.0 mm/d) in 2005; estimates were within 1% of the BREBS-measured ETc in 2006, but with a greater RMSD (1.3 mm/d). T was 65%-70% of ETc and E was ~30% of ETc in 2005; T was 60%-65% of ETc and E was 35% of ETc in 2006. ABSTRACT. Measured crop coefficients, especially for subsurface drip-irrigated (SDI) cropping systems, are rare, but they play a critical role in estimating crop evapotranspiration (ET c ) (or latent heat flux, LE) for effective irrigation management, water resources planning, and many other purposes. Surface energy balance variables [ET c , sensible heat flux (H), soil heat flux (G), incoming shortwave radiation (R s ), net radiation (R n ), surface albedo (a)], plant variables [leaf area index (LAI), plant height], and climate variables were measured for SDI-irrigated maize using an advanced Bowen Ratio Energy Balance System (BREBS). Crop growth stage-specific single and basal K c curves (K c and K cb , respectively) were developed as a function of cumulative growing degree days (SGDD). The FAO56 method was employed to partition ET c into transpiration (T) and evaporation (E). The seasonal average BREBS energy balance closure (LE+H = R n - G) error was only 2.63 W/m 2 with a low maximum difference of 19.3 W/m 2 in 2005, and the average error was 3.62 W/m 2 with a seasonal maximum difference of 18.4 W/m 2 in 2006. ET c for a given growth stage varied within the season and for the same growth stage between the seasons. BREBS-measured seasonal ET c was 667 mm in 2005 and 571 mm in 2006. The field-average crop water productivity was 2.5 kg/m 3 in 2005 and 2.8 kg/m 3 in 2006. The maximum growth-stage-specific ET c was measured during the R1-Silking to R2-Blister stages in 2005 (107 mm), while it was 79.8 mm between the same growth stages in 2006. In 2005, during the vegetative stage (V1 to Tasseling), ET c was 202 mm, which accounted for 30% of the seasonal ET c , and ET c for the reproductive stage [Tasseling to Physiological Maturity (PM)], was 419 mm (accounting for 63% of the total ET c , 667 mm). In 2006, both vegetative and reproductive stages had similar ET c . ET c during the vegetative stage (V1 to Tasseling) was 267 mm (accounted for 47% of the seasonal ET c , 571 mm), and ET c for the reproductive stage (Tasseling to PM) was 266 mm (also accounted for 47% of the seasonal ET c ). On a two-season average, R1-Silking to R2-Blister stage had the highest ET c (93.4 mm), followed by V10 to V12 stage (73.1 mm), Tasseling to R1-Silking (56.3 mm), and R2-Blister to R3-Milk, R4-Dough to R5-Dent, and V12 to V18-Tasseling stages all having similar ET c (53-55 mm). From planting to V1, ET c (46 mm in 2005 and 38.2 mm in 2006) accounted for 7% and 6% of the seasonal ET c , respectively. K c values exhibited strong dependence on SGDD in both seasons. The growth stage-specific K c values differed between the growing seasons more than the interannual variation for the same stage. The maximum K c was 1.20 during the R1-Silking to R2-Blister stage as in 2005, whereas the maximum was observed during the V12 to V18-Tasseling and R1-Silking stages as 1.25 and 1.24, respectively, in 2006. The relationship between LAI and K c was strong and linear in both seasons. In 2005, the FAO56 method overestimated BREBS-measured ET c by 9.3% with a root mean squared difference (RMSD) of 1.0 mm/d, while its estimates in 2006 were within 1% of the BREBS-measured ET c , but with a higher RMSD (1.28 mm/d). The FAO56 method overestimated BREBS-measured seasonal ET c by 102 mm (15%) and 61 mm (10.3%) in 2005 and 2006, respectively. In 2005, maize T ranged from near zero during emergence and during the late season (during PM and leaf senescence) to 9 mm/d at R1-Silking and R2-Blister stage; and ranged from near zero to 9.7 mm/d during R1-Silking stage in 2006. In both seasons, T exhibited its maximum values from about V10 stage R4-Dough stage. Seasonal E was 116 mm in 2005 and 124 mm in 2006. On a seasonal average basis, T was 65%-70% of ET c and E was ~30% of ET c in 2005; T was 60%-65% of ET c and E was 35% of ET c in 2006. Data and information presented in this research can be invaluable for better management of irrigated maize production while maximizing the effectiveness of planning, allocating, and using water resources.