Marvin E. Jensen

Estimating Soil Moisture Depletion from Climate, Crop and Soil Data

TREMENDOUS international scientific neffort has been expended on nevaporation and transpiration problems nduring the past decade as evidenced nby hundreds of technical publications, nand numerous conferences. However, nuse of this scientific achievement by nagriculturalists, project planners and noperators of irrigation farms has lagged nbehind technological advancements. nThe lag in adaptation of new technology nby the user can be partly attributed nto a lack of time, technical ntraining and experience in meteorology, nphysics and agronomy

Beyond irrigation efficiency

Parameters for accounting for water balance on irrigation projects have evolved over the past century. Development of the classic term irrigation efficiency is summarized along with recent modifications such as effective irrigation efficiency. The need for terms that describe measurable water balance components of irrigated agriculture is very important, as demands and competition for available renewable water supplies continue to increase with increasing populations. Examples of irrigation efficiency studies conducted during the past few decades are summarized along with related irrigation terminology. Traditional irrigation efficiency terminology has served a valid purpose for nearly a century in assisting engineers to design better irrigation systems and assisting specialists to develop improved irrigation management practices. It still has utility for engineers designing components of irrigation systems. However, newer irrigation-related terminology better describes the performance and productivity of irrigated agriculture. On a river-basin level, improved terminology is needed to adequately describe how well water resources are used within the basin. Brief suggestions for improving irrigation water management are presented.

Development and Evaluation of Evapotranspiration Models for Irrigation Scheduling

ADSTRACT EVAPOTRANSPIRATION (ET) data for irri-gated crops in southern Idaho were used to de-velop relationships for estimating net radiation and potential ET for the USDA-ARS Computerized Irri-gation Scheduling Program. ET estimated with the initial relationships compared well with recent mea-surements obtained with two sensitive weighing lysim-eters. The average daily measured Et for alfalfa for 128 days when there was full cover was 7.23 mm, while the average daily estimated ET was 7.15 mm. Crop curve relationships were developed from the ET results for snap beans (Phaseolus vulgaris L). The depletion of soil water was predicted for two years of irrigated beans with the scheduling program using the improved crop curves and compared with the mea-sured as a test of its performance. The standard devia-tion of the difference between predicted and measured was about 0.95 mm/day from planting until harvest. The results also demonstrated the importance of ob-taining representative meteorological data for irrigation scheduling.

The Role of Evapotranspiration Models in Irrigation Scheduling

ADSTRACT MOST evapotranspiration (ET) models are based on physical principles controlling evaporation and the conservation of mass and energy, and use daily climatic data. ET models coupled with irrigation models are valuable tools because they enable trained and ex-perienced irrigation specialists to provide irrigation scheduling services at a reasonable cost. Estimated standard deviations of mean daily ET for 1- to 30-day periods (S^-) at Akron, Colorado; Davis, California; Kimberly, Idaho; and Lompoc, CA varied from 0.9 to 1.3 mm/day. Sgy decreased to 0.4 to 0.7 mm/day for 15- to 30-day periods. Standard errors of ET estimates (Sy.x) with a com-bination equation based on 243 days of data from Kimberly, ID were normally distributed. The Svx for daily values was 1.0 mm/day. The Sv#x decreased in-versely with the square root of the number of days for up to 30 days and was similar to those reported for other areas using models that operate on daily climatic data. A summary of factors affecting confidence levels in irrigation scheduling is presented along with the ex-pected standard deviations. Generally, the error in esti-mating irrigation applications exceeds estimated ET errors. The error in measuring soil moisture is gener-ally smaller than estimated ET and irrigation applica-tion errors.

Changes in Climate and Estimated Evaporation Across a Large Irrigated Area in Idaho

G round level climatic measurements were taken along a 50 km transect going from dry sagebrush land into the center of a large irrigated area in southern Idaho. Measurements in May, when the desert area was dry, indicated that climatic changes across the transect were minimal. In August, when the desert was obviously very dry, air temperatures decreased, vapor pressure increased, and windspeed was reduced about 40 percent within the irrigated area. The results demonstrate that any weather service agency or group must consider the distance from dry surroundings when selecting sites that are to be representative of climatic conditions over irrigated fields.

Evapotranspiration and Soil Moisture-Fertilizer Interrelations with Irrigated Grain Sorghum in the Southern High Plains

In 1956 and 1957 grain sorghum represented n37 percent of the harvested crop acreage in Texas. The largest concentrated area of sorghum nis in the High Plains where the proportion of nsorghum irrigated increased greatly during the ndrought years of the 1950s. In 1959 the monetary nvalue of irrigated grain sorghum was estimated nto be about

ASCE's Standardized Reference Evapotranspiration Equation

100 million. nAs reported by the U.S. Census of Agriculture nthe acreages of irrigated grain sorghum harvested nin the 42-county High Plains area in 1950, 1954, nand 1959 were 387,000, 1 1 006,000, and 1,224,000 nacres, respectively. The irrigated grain sorghum nacreage in the eight counties—Castro, Deaf nSmith, Floyd, Hale, Lamb, Lubbock, Farmer, nand Swisher—represented over 80 percent of the ntotal in the High Plains in 1954 and 1959.

Irrigated Agriculture at the Crossroads

The ASCE Evapotranspiration in Irrigation and Hydrology Committee (ASCE-ET) is recommending, for the intended purpose of establishing uniform evapotranspiration (ET) estimates and transferable crop coefficients, two Standardized Reference Evapotranspiration Surfaces : (1) a short crop (similar to grass) and (2) a tall crop (similar to alfalfa), and one Standardized Reference Evapotranspiration Equation . The standardized equation is derived from the ASCE-Penman Monteith equation, by simplifying several terms within that equation. The standardized equation, with appropriate constants provided in an accompanying table, is used to calculate evapotranspiration for the standardized short reference (ET os ) and/or evapotranspiration for the standardized tall reference (ET rs ). One constant is in the numerator and one is in the denominator. The constant in the right-hand side of the numerator (C n ) is a function of the time step and aerodynamic resistance (i.e., reference type). The constant in the denominator (C d ) is a function of the time step, bulk surface resistance, and aerodynamic resistance (the latter two terms vary with reference to type, time step, and daytime/nighttime).

Programming Irrigation for Greater Efficiency

Dr. Pereira set the stage and outlined the goals for this NATO Advanced Research Workshop on Sustainability of Irrigated Agriculture. He and his co-conveners and the Portugal National Organizing Committee identified and assembled world renown irrigation specialists. The International Commission on Irrigation and Drainage (ICID) family provided many participants. In their papers, participants were asked to review the state-of-the-art of the science and technology of irrigation for the readers. The primary purpose, however, is to integrate science and technology to enable improving irrigation system performance and to provide recommendations for future irrigation research. Irrigated and rainfed agriculture must be integrated to meet future food requirements. Therefore, a parallel effort is needed on water conservation and management for rainfed areas.

Sprinkler Irrigation Spray Temperatures

Programming irrigation for greater efficiency implies nmaximizing water use efficiency. However, though this in nitself is an important objective for water deficient areas, nthe farm manager or owner is more interested in maximizing nhis net income by optimum use of irrigation water, fertilizers, nand other inter-related inputs. Fortunately, if maximum nnet income is achieved, optimum water use efficiency nusually has also been attained. More progress toward ngreater efficiency can be expected by working toward the ngoal of greater net income rather than greater water use nefficiency, per se.