José O. Payero

COMPARISON OF ELEVEN VEGETATION INDICES FOR ESTIMATING PLANT HEIGHT OF ALFALFA AND GRASS

A great variety of vegetation indices, derived from remote sensing measurements, are commonly used to characterize the growth pattern of cropped surfaces. In this study, multispectral canopy reflectance data were obtained from grass (Festuca arundinacea) and alfalfa (Medicago sativa L.) at Kimberly, Idaho, with the purpose of comparing the performance of 11 vegetation indices for estimating plant height of these two structurally different crop canopies. An additional purpose was to develop quantitative relationships between plant height and the different vegetation indices, which could be used to estimate plant height from remote sensing inputs. For alfalfa, good logistic growth relationships between plant height and all the different vegetation indices were found. The relationship resulted in r2 > 0.90 for all the vegetation indices, and r2 > 0.97 for most of them. While all the vegetation indices were very sensitive to changes in plant height at the beginning of the growing cycle, the Normalized Difference Vegetation Index (NDVI), the Infrared Percentage Vegetation Index (IPVI), and the Transformed Vegetation Index (TVI) became insensitive to additional plant growth when alfalfa reached heights of 0.45, 0.40, and 0.45 m, respectively. All the other vegetation indices performed reasonably well for the entire range of alfalfa plant heights considered in this study ( 2 ~ 0.76).

Field Scale Limited Irrigation Scenarios for Water Policy Strategies

Approaches to reducing irrigation inputs to crops have been studied for the past 50 to 60 years in research settings. Fewer efforts have been made to document limited irrigation responses over a number of seasons on commercial fields. This study compared farm-based irrigation management (FARM) with best management practices (BMP), late initiation of irrigation (LATE), and a restricted allocation (ALLOC). These irrigation management strategies each occupied 1/8 of a center pivot system in southwest Nebraska in continuous corn production, on four cooperating farms, which were replicated at the same sites for 3 to 6 years. Irrigation variables were achieved by irrigating or not irrigating, or by speeding up or slowing down the center pivot. When the grain yields and irrigation amounts were normalized each year using the FARM treatment as the basis, on average for three of four locations, the BMP treatment yielded equal to the FARM treatment, the LATE treatment yielded 93% of the FARM treatment and the ALLOC yielded 84% of the FARM treatment. At the same time, it took 76% and 57% of the water for the LATE and ALLOC treatments, respectively, to achieve these yields. The adjusted gross returns (yield . price – irrigation treatment costs) of the irrigation treatments were analyzed for each location. When the gross returns were normalized using the FARM treatment as the basis, FARM and BMP returns were equal across combinations of high and low input commodity prices and pumping costs. The LATE treatment gross return was 95% of FARM return. The gross return for the ALLOC treatment was 85% to 91% of the FARM treatment. The higher the water costs, the lower the difference between the highest and lowest returning water treatments. Relationships between evapotranspiration and grain yield were developed for two sites over the limited range of water applications of the projects. Regressions indicated more variability between the commercial field data and research plot environments. Much of this difference may have been due to yearly replication in this study rather than plot-to-plot replication in the research center study. Yield and irrigation data were normalized on the basis of the FARM treatment. Normalized yield – irrigation results over years and locations for three of the four locations showed declining yields as irrigation decreased. The same regression was used to normalize the locations with soil textures from fine sand to sandy loam, which suggested that the three locations behaved similarly with respect to the management treatments.

GUIDELINES FOR VALIDATING BOWEN RATIO DATA

For a variety of reasons, the measurement of latent heat flux using the Bowen ratio method can sometimes result in erroneous data. This study provides guidelines for detecting erroneous Bowen ratio data and illustrates the application of these guidelines by comparing Bowen ratio and lysimeter data collected over grass and alfalfa in southern Idaho. Errors in net radiation were detected by comparing measured with theoretical values. However, it was found that good theoretical procedures to validate soil heat flux data are lacking. Only empirical equations mainly used for remote sensing applications to obtain estimates close to noontime are available. Extremely inaccurate latent heat fluxes were easily filtered out by rejecting data when the calculated Bowen ratio (β) values were close to -1. A simplified procedure was proposed to reject fluxes with the wrong sign, and three different equations were used successfully to detect the occurrence of condensation inside the type of measurement system used in the study. Guidelines to assure adequate fetch are provided. Fetch did not affect the measured fluxes in this study, which may have been due to the similarity in surface properties between the crops under study and those in the surrounding fields.

Standardized ASCE Penman-Monteith: Impact of sum-of-hourly vs. 24-hour timestep computations at reference weather station sites

The standardized ASCE Penman-Monteith (ASCE-PM) model was used to estimate grass-reference evapotranspiration (ETo) over a range of climates at seven locations based on hourly and 24 h weather data. Hourly ETo computations were summed over 24 h periods and reported as sum-of-hourly (SOH). The SOH ASCE-PM ETo values (ETo,h,ASCE) were compared with the 24 h timestep ASCE-PM ETo values (ETo,d) and SOH ETo values using the FAO Paper 56 Penman-Monteith (FAO56-PM) method (ETo,h,FAO). The ETo,h,ASCE values were used as the basis for comparison. The ETo,d estimated higher than ETo,h,ASCE at all locations except one, and agreement between the computational timesteps was best in humid regions. The greatest differences between ETo,d and ETo,h,ASCE were in locations where strong, dry, hot winds cause advective increases in ETo. Three locations showed considerable signs of advection. Some of the differences between the timesteps was attributed to uncertainties in predicting soil heat flux and to the difficulty of ETo,d to effectively account for abrupt diurnal changes in wind speed, air temperature, and vapor pressure deficit. The ETo,h,FAO values correlated well with ETo,h,ASCE values (r2 > 0.997), but estimated lower than ETo,h,ASCE at all locations by 5% to 8%. This was due to the impact of higher surface resistance during daytime periods. Summing the ETo values over a weekly, monthly, or annual basis generally reduced the differences between ETo,d and ETo,h,ASCE. The differences suggest that using ETo,d rather than ETo,h,ASCE would result in underestimation or overestimation of ETo. Summing the ETo,d values over multiple days and longer periods for peak ETo months resulted in inconsistent differences between the two timesteps. The results suggest a potential improvement in accuracy when using the standardized ASCE-PM procedure applied hourly rather than daily. The hourly application helps to account for abrupt changes in atmospheric conditions on ETo estimation in advective and other environments when hourly climate data are available.

RESPONSE OF SOYBEAN TO DEFICIT IRRIGATION IN THE SEMI-ARID ENVIRONMENT OF WEST-CENTRAL NEBRASKA

Several factors, including multi-year drought, declining aquifer levels, and new water regulations, are contributing to reduced availability of irrigation water in the semi-arid area of west-central Nebraska. Since many farmers in this area do not have enough water to meet the seasonal water requirements of crops like corn and soybean, maximizing yield produced per unit of water under deficit irrigation conditions is becoming increasingly important. This study was conducted to quantify the grain yield response of soybean [Glycine max (L.) Merr.] to deficit irrigation, and to determine which seasonal water variables correlated best to soybean grain yield under deficit irrigation. The study was conducted during 2002 at Curtis, and 2003 and 2004 at North Platte, Nebraska. Nine deficit irrigation treatments, including different irrigation amounts and timings, were studied in 2002 and 2003, and eight treatments were studied in 2004. Soybean grain yields across years and sites were best related to the seasonal ratio of the actual crop evapotranspiration and the crop evapotranspiration when soil water was not limiting (ETd/ETw), and to the seasonal ratio of actual crop transpiration and crop transpiration when soil water was not limiting (Td/Tw). Both of these seasonal ratios were linearly related to grain yield with R2 = 0.91 when combining data for all seasons. The crop water productivity (CWP) (yield per unit of seasonal ETd) linearly increased with both ETd/ETw (R2 = 0.72) and Td/Tw (R2 = 0.72), but was best correlated to the daily positive difference between the actual and the theoretical fraction of total available soil water in the root zone that can be depleted before crop water stress occurred, accumulated for the entire season (seasonal pdiff) (R2 = 0.77). A linear relationship between the cumulative ETw and fraction of season (function of days after emergence) was found. This relationship developed for a given location could be used to extrapolate seasonal ETw for in-season irrigation management. Poor correlation was found between CWP and other variables such as total irrigation, rain + irrigation, and total water. The results of this study can provide useful information for soybean irrigators to make better management decisions under deficit irrigation conditions.

Variable upper and lower crop water stress index baselines for corn and soybean

Upper and lower crop water stress index (CWSI) baselines adaptable to different environments and times of day are needed to facilitate irrigation scheduling with infrared thermometers. The objective of this study was to develop dynamic upper and lower CWSI baselines for corn and soybean. Ten-minute averages of canopy temperatures from corn and soybean plots at four levels of soil water depletion were measured at North Platte, Nebraska, during the 2004 growing season. Other variables such as solar radiation (Rs), air temperature (Ta), relative humidity (RH), wind speed (u), and plant canopy height (h) were also measured. Daily soil water depletions from the research plots were estimated using a soil water balance approach with a computer model that used soil, crop, weather, and irrigation data as input. Using this information, empirical equations to estimate the upper and lower CWSI baselines were developed for both crops. The lower baselines for both crops were functions of h, vapor pressure deficit (VPD), Rs, and u. The upper baselines did not depend on VPD, but were a function of Rs and u for soybean, and Rs, h, and u for corn. By taking into account all the variables that significantly affected the baselines, it should be possible to apply them at different locations and times of day. The new baselines developed in this study should facilitate the application of the CWSI method as a practical tool for irrigation scheduling of corn and soybean.

Large-scale on-farm implementation of soil moisture-based irrigation management strategies for increasing maize water productivity

Irrigated maize is produced on about 3.5 Mha in the U.S. Great Plains and western Corn Belt. Most irrigation water comes from groundwater. Persistent drought and increased competition for water resources threaten long-term viability of groundwater resources, which motivated our research to develop strategies to increase water productivity without noticeable reduction in maize yield. Results from previous research at the University of Nebraska-Lincoln (UNL) experiment stations in 2005 and 2006 found that it was possible to substantially reduce irrigation amounts and increase irrigation water use efficiency (IWUE) and crop water use efficiency (CWUE) (or crop water productivity) with little or no reduction in yield using an irrigation regime that applies less water during growth stages that are less sensitive to water stress. Our hypothesis was that a soil moisture-based irrigation management approach in research fields would give similar results in large production-scale, center-pivot irrigated fields in Nebraska. To test this hypothesis, IWUE, CWUE, and grain yields were compared in extensive on-farm research located at eight locations over two years (16 site-years), representing more than 600 ha of irrigated maize area. In each site-year, two contiguous center-pivot irrigated maize fields with similar topography, soil properties, and crop management practices received different irrigation regimes: one was managed by UNL researchers, and the other was managed by the farmer at each site. Irrigation management in farmer-managed fields relied on the farmers’ traditional visual observations and personal expertise, whereas irrigation timing in the UNL-managed fields was based on pre-determined soil water depletion thresholds measured using soil moisture sensors, as well as crop phenology predicted by a crop simulation model using a combination of real-time (in-season) and historical weather data. The soil moisture-based irrigation regime resulted in greater soil water depletion, which decreased irrigation requirements and enabled more timely irrigation management in the UNL-managed fields in both years (34% and 32% less irrigation application compared with farmer-managed fields in 2007 and 2008, respectively). The average actual crop evapotranspiration (ETC) for the UNL- and farmer-managed fields for all sites in 2007 was 487 and 504 mm, respectively. In 2008, the average UNL and average farmer-managed field had seasonal ETC of 511 and 548 mm, respectively. Thus, when the average of all sites is considered, the UNL-managed fields had 3% and 7% less ETC than the farmer-managed fields in 2007 and 2008, respectively, although the percentage was much higher for some of the farmer-managed fields. In both years, differences in grain yield between the UNL and farmer-managed fields were not statistically significant (p = 0.75). On-farm implementation of irrigation management strategies resulted in a 38% and 30% increase in IWUE in the UNL-managed fields in 2007 and 2008, respectively. On average, the CWUE value for the UNL-managed fields was 4% higher than those in the farmer-managed fields in both years. Reduction in irrigation water withdrawal in UNL-managed fields resulted in

ESTIMATING SOIL HEAT FLUX FOR ALFALFA AND CLIPPED TALL FESCUE GRASS

32.00 to

AQUAMAN: a web-based decision support system for irrigation scheduling in peanuts

74.10 ha-1 in 2007 and

Agronomic and economic evaluation of irrigation strategies on cotton lint yield in Australia

44.46 to