Sign up for email alerts of new Fluid Journal issues!
Fluid Journal : Late Spring 2013
8 The Fluid Journal Late Spring 2013 current irrigated cropland into dryland agriculture. However, the option has an unavoidable tradeoff of a 55% reduction in grain yield and much greater year-to-year yield variability as shown by comparison of yields and yield variability of rain-fed and irrigated maize in the Tri-Basin NRD. Assuming elimination of irrigated maize production, the amount of additional maize area (in addition to all existing maize land area in Tri-Basin NRD) to replace this lost production would depend on yield level in the new production area. For example, based on current average rain-fed yields, replacement would require 124,170 ha in Nebraska, 90,517 ha in Iowa, or 276,722 ha in Brazil. Additional land requirements, GHG emissions from land use change, and GHG emissions from crop production on this newly converted land would offset apparent benefits of expanding low-input/low- yield rain-fed maize at the expense of irrigated maize in the Tri-Basin NRD. Given concerns about land use, the most promising avenue to reduce GHG emissions, without significant impact on productivity, appears to be through improvements in input use efficiency of current irrigated maize systems. Among irrigated maize fields in the Tri-Basin NRD, lack of correlation between irrigated yields and energy input or GWP in all years and three- and four-fold greater variation in energy inputs and GWP than observed variation in grain yield (Figure 3, A and B) suggests substantial scope to improve energy balance and to reduce GWP of irrigated maize without affecting productivity. Differences in both applied irrigation and magnitude of N surplus explained 57% of the variation in GWP. Therefore, achieving greater NUE and water productivity through better management of applied N and irrigation water would be a most effective way for increasing energy yield and reducing GHG emissions. Analysis of farmer's data indicated that values of NER and GWP higher and lower than 6.5 and 218 kg of CO2eMg-1 of grain, respectively, can be set as reasonable energetic and environmental targets for irrigated maize (Figure 3 A and B). In fact, achieving high yield with large energy inputs and high input use efficiency resulted in a strong negative correlation between GWPi and NEY (Figure 3C). This finding is consistent with results from a previous life cycle assessment for maize- enhanced systems. There is, however, an important distinction between analyses based on Tri-Basin NRD irrigated maize data and previously published data. In the present study, HEY and GWPi were calculated based on (1) maize yield and input data collected during a recent 3-year time interval (2005 to 2007) across a large number of farmer fields, (2) the most recent embodied energy values for inputs to estimate energy balance and GHG emissions, and (3) the N-surplus approach to estimate soil N2O emissions. In contrast, previous studies relied on national or statewide aggregated yield and applied input statistics and the IPCC-N input approach to estimate soil N2O emissions. Also, the embodied-energy and GHG-emission values for specific inputs were not consistent across these previous studies and in some cases the values used are now obsolete and/or unrepresentative compared with current crop management practices and manufacturing efficiencies. The impact from adoption of best management practices, compared with current average management, on energy use and GWP was evaluated for irrigated maize in the Tri-Basin NRD (Table 2). Best management practices included use of low-pressure pivot irrigation, improved irrigation pump performance rating (PPR), use of electrical power for irrigation water pumping rather than diesel or natural gas, fine-tuning of irrigation timing, and better N fertilizer management. Taken together, adoption of these management practices would result in a 25% and 21% reduction in energy Figure 4. Average (±SE) energy input rate, net energy yield, net energy ratio, and GWPi of irrigated maize under different combinations of irrigation system (pivot, surface), crop rotation (maize after maize [M-M] or maize after soybeans [S-M]), and tillage method (conventional [CT]; reduced till [RT]). Maize grain yields (Mg. ha-1) are shown above bars in Middle Upper. All values are 3-y (2005- 2007) means. Differences (r) and t test significance for selected comparisons between factor levels are shown (n.s., not significant).
Early Spring 2013