Rapid field evaluation of drip and microspray distribution uniformity

The Cal Poly ITRC irrigation evaluation programs have been widely used to assess the global distribution uniformity (DU) of drip and microsprayer irrigation systems. The field procedures and formulas used in the program are presented in this paper. The system DU is estimated by mathematically combining the component DU values. DU components include pressure differences, other causes (such as manufacturing variation, plugging, and wear), unequal drainage, and unequal application rates. Results are presented from evaluations by several entities, including Cal Poly ITRC. Cal Poly evaluations of 329 fields provided an average DU_lq of 0.85 for drip and 0.80 for microspray. Approximately 45% of the non-uniformity was due to pressure differences, 52% was due to other causes, 1% due to unequal drainage, and 2% due to unequal application rates. The data show that with good design and management, it is possible to have high system DU values for at least a 20-year system life.     

Infiltration and runoff as affected by pitting,mulching and sprinkler irrigation

Summary Changes in infiltration and runoff caused by pitting and mulching under sprinkler irrigation were studied on two soil types. Pitting or diking was done with an implement called a dammer-diker. Five soil treatments were applied: shallow and deep dammer-diker, shallow dammer-diker with mulch, bare, and a mulched soil, combined with two water application rates. Total water infiltration and runoff varied during the experiment. Runoff decreased with area of water storage provided by the pits and the less water was applied. Mulch treatments also reduced runoff. Surface water storage decreased during the season. Changes in soil physical properties due to pitting were more important in controlling runoff than surface water storage.The effective saturated hydraulic conductivity of the soil progressively decreased through the season for all soil treatments and water application rates.A model was developed to simulate the effect of pits on runoff. On a silt loam soil, simulated percent runoff and accumulated runoff over time for the bare and pitted treatments agreed closely to measured values. The agreement of simulated to measured runoff for a silty clay loam soil was not as good probably because of cracking which the simulation model did not take into account.     

Yield and vegetative growth as related to plant water potential of cotton irrigated with a moving sprinkler system at different frequencies and wetting depths

Summary Seed-cotton yield, yield components and vegetative growth were determined under different irrigation frequencies and wetting depths with a self-propelled moving-irrigation-system (MSIS) in 1986 and 1987. Irrigation timing was determined in both years by pre-irrigation, mid-day plant water potential (_w). The amount of water to be applied was determined by measuring the soil moisture deficit. In 1987, the effect of a change from one irrigation frequency and wetting depth to another at mid-flowering was also examined. Linear responses of relative seed-cotton yield to the amount of evapotranspiration (ET) were found for both years with similar slopes but different intercepts. Significant positive regressions were obtained between pre-irrigation plant _w and relative seed-cotton yield, and vegetative growth during the linear growth stage. Seed-cotton yield was affected by both wetting depth and pre-irrigation plant _w. The deeper the irrigation the higher was the seed-cotton yield for each pre-irrigation plant _w. Irrigation frequencies which maintained plant _w above -1.5 MPa during vegetative growth, flowering and boll-filling resulted in maximum production. The boll filling stage appeared to be a very sensitive one, as boll weight was found to be the main yield component responding to irrigation treatments. At a wetting depth of 120 cm, higher seed-cotton yields were obtained than at a more shallow wetting. Different irrigation managements resulted in different turgor potentials (_t) mainly during mid-day. Both leaf water vapour conductance and net assimilation rate were sensitive to leaf _w.Contribution from the Agricultural Research Organization, The Volcani Center, Bet Dagon, Israel, No. 2903-E, 1990 series. Research was supported by the U.S.-Israel Binational Agric. Res. and Develop. Fund.     

Potassium induced improvement of yield response in barley exposed to soil water stress

Summary The interaction of different K status of barley plants (Hordeum vulgare, L.) and water stress on yield and water relations was studied. The plants which were cultivated outdoor in pots and supplied with 0.8, 5.0, 8.5 or 12.0 g K per pot, as KCl, were subjected to increased soil water stress during the early grain filling stage.The water content of the flag leaf tissue was significantly increased from 3.1 to 4.1 g H₂O/g D.M. (dry matter) by K application resulting in maintenance of similar leaf osmotic potentials (–1.5 MPa) at all K levels prior to onset of water stress (Table 2). At the lowest K level Ca contributed essentially to maintenance of the cell osmotic potential (Fig. 2).In fully watered plants grain yield at the lowest K level was reduced 20% (Fig. 5 a) due to a decrease in the number of tillers with ears per plant (Fig. 5 b) and to early commencement of maturity processes (Table 3).Water stress caused grain yield reductions between 15 and 50%. However, by increase of K application yield was maintained to the greatest degree in high K plants (Fig. 5 a) due to improved water status in these plants during the drying cycle (Fig. 4). The production of above ground dry matter (top D.M.) during the grain filling period and the grain yield were highly correlated with the leaf water content at the end of the drying cycles (Fig. 6). The greater yield in high K plants was associated with prolongation of the grain filling period by up to 7 days (Table 3) and with an increase in grain weight by up to 20% (Fig. 5 b) as compared with low K plants. Preanthesis reserves contributed up to 52% of grain yield at low K levels (Fig. 5 c) reducing differences in grain yield between the K levels.Abbreviations RWC predawn relative water content - predawn leaf osmotic potential - WUE water use efficiency - R preanthesis reserves - ear D.M. increase in ear D.M. during the grain filling period - top D.M. increase in top D.M. during the grain filling period - SD standard deviation - LSD least significant difference     

Effect of soil water osmotic potential on growth and water relationships in barley during soil water depletion

Summary Barley plants (Hordeum distichum, L., cv. Zita) grown in a sandy soil in pots were adjusted during a pretreatment period of 5 days to three levels of soil water osmotic potential by percolating 61 of a nutrient solution with additional 0, 22.3 and 44.6 mM KCl. A drying cycle was then started and the plants were harvested when the soil water matric potential had decreased to –1.4 MPa, respectively 6, 7 and 8 days later.No significant differences in dry matter yields, transpiration coefficients and wilting percentages were found between treatments.During the drying cycle leaf water potential ( _l ) decreased concomitantly with decrease in soil water potential ( _s ) with almost constant and similar differences ( _l – _s ) for all treatments despite differences in levels of potentials. The concomitant decrease in leaf osmotic potential () was due partly to dehydration (58%) and partly to increase in leaf solute content (42%) independent of treatment. The part of total osmotic solutes due to K decreased relatively during the drying cycle.Close relationships were found between and _l as functions of relative water content (RWC). Identical curves for the two levels of salt treatment agree with similar concentrations of K, Cl, and ash found for salt treated plants indicating that maximum uptake of macro nutrients may have been reached.During the main part of the drying cycle the turgor potential as function of RWC was higher and decreased less steeply with decreasing RWC in the salt treated than in the non-salt treated plants.In the beginning of the drying cycle additions of KCI lowered the transpiration rates of the salt treated plants resulting in a slower desiccation of the soil and hence an increased growth period. A delay in uptake from a limited soil water supply may be advantageous during intermittent periods of drought.     

Pressurised irrigation technologies for smallholders in developing countries – a review

The findings of a study of factors influencing the uptake of pressurised irrigation technologies by smallholders in developing countries are presented. The paper reviews the physical and technical characteristics that determine their suitability for use by smallholders. It also identifies a range of pre-conditions relating to water availability, institutional support and economic opportunity that must be satisfied before smallholders will adopt even low-technology pressurised irrigation systems.The review demonstrates that where physical, economic and institutional conditions are right some forms of pressurised modern irrigation technology permit smallholder irrigation of high value crops where surface irrigation would be inappropriate. However, the paper warns against the danger of wide-scale promotion of such technologies without considering the issues of institutional and technical support. Where pressurised systems are promoted to increase water use efficiency it is essential that they be well designed, installed and operated for savings to be realised.     

Leaching of soluble salt during infiltration and redistribution

Summary The paper reports an experimental study of miscible displacement of soluble salts during infiltration and redistribution of water in vertical, homogeneous columns of sandy, sandy loam and clay soils with initially uniform salt and moisture contents. Calcium chloride, mixed uniformly in initially dry and moist soils, was leached with water under transient and steady infiltration conditions. The salt and water profiles were determined immediately following infiltration and after matching total infiltration and redistribution times. Irrespective of different flow conditions and soil types, the centre of mass of salt front coincided with the piston front that would exist given perfect displacement of water initially present in the soil by the water being infiltrated (piston-flow model). Furthermore the advance of centre of mass of salt front was independent of the water application rate and initial soil water content in all soils following both infiltration and redistribution.     

Irrigation scheduling in a mature peach orchard using tensiometers and dendrometers

Three trickle irrigation schedules, two of which were scheduled according to soil water potential ( soil) (tensiometer method) and daily stem contraction (DSC) (dendrometer method) respectively and the other one was a schedule of restricted water supply, were applied to a mature peach orchard.The annual water application based on soil was greater than that based on DSC. However, tree growth, fruit size and leaf water potential (leaf) on the trees in the dendrometer scheduling plot did not differ from those in the tensiometer scheduling plot while the premature fruit drop and fruit bud initiation were greatly different. The restricted water supply treatment limited significantly both tree and fruit growth. In addition, the lower leaf was observed on the trees in this plot.Further study shows that use of the dendrometer method for scheduling irrigation satisfies the water needs of the plant and that the tensiometer method is less accurate.Abbreviations leaf leaf water potential - soil soil water potential - DSC daily stem contraction - LVDT linear variable displacement transducer - PET potential evapotranspiration     

Effects of salinity on germination,seedling growth,and yield of melons

Summary Four melon (Cucumis melo L.) cultivars were tested for salt tolerance at germination, seedling growth stages, and plant maturation. Noy Amid was the most tolerant during germination, achieving 56% germination in 15,000 mg/l NaCl solution. However, this cultivar and Eshkolit Ha 'Amaqim were relatively sensitive during the first 4 days' growth of the radicle and the hypocotyl, and the first 3 weeks' development of the seedling. Their yields were reduced under saline as compared with non-saline field conditions. In contrast, Honey Dew and Rochet had little or no germination in 15,000 mg/l NaCl but showed salt tolerance during seedling growth stages. Yield of Honey Dew was unaffected by saline field conditions, and that of Rochet was not significantly reduced from the non-saline control. Thus, selection for salt tolerance in melons appears feasible during early vegetative growth stages but not during germination.Contribution No. 1032-E, 1984 series, from the Agricultural Research Organization, Bet Dagan, Israel     

Effects of alternative water distribution rules on irrigation system performance: a simulation analysis

Based on a simulation model reflecting physical and economic conditions typically found in rice irrigation systems in Asia, the irrigation performance implications of alternative water distribution rules for dry season irrigation are evaluated under varying degrees of water shortage. The rules examined reflect differing water distribution strategies designed either to maximize conveyance efficiency, economic efficiency, or equity; or to achieve a balance between efficiency and equity objectives. Irrigation performance is evaluated using several efficiency measures reflecting the physical, agronomic and economic productivity of water, and one measure of equity. Economic efficiency and equity among farmers within the portion of the irrigation system that is on in any given season are shown to be complementary, and not competing objectives. Economic efficiency and equity among all farmers within the command area of the irrigation system are largely complementary strategies at the lower levels of water shortage, but with increasing shortage, significant tradeoffs develop between these objectives. An operational rule for water distribution under a goal of maximizing economic efficiency is developed, and the data requirements for its implementation are shown to be modest. Under the model's assumed conditions of dry season rice production dependent solely on surface irrigation for water, the distribution strategy designed to maximize conveyance efficiency results in only modestly lower levels of economic efficiency and equity than could be achieved by the strategy designed to maximize economic efficiency.     

Water management in tertiary units in the Fayoum,Egypt

The irrigation system in The Fayoum is designed for a continuous supply of water (24 h a day, 7 days a week) to rotational units, which vary in size from about 20 to 500 fe. The Fayoum Irrigation Department is responsible for the delivery of irrigation water to the rotational units. Within the units, the farmers rotate the water in a seven days rotation.One of the characteristics of present water management of The Fayoum is the non-uniformity of the division of flow over the main canals. The purpose of this paper is: (i) to study the effects of this non-uniformity by comparing water management in tertiary units in an area with a (more than) sufficient water supply to an area subject to some water shortage; (ii) to discuss implications of the present tertiary unit water management for the water management of the entire The Fayoum.The findings are that: (i) the non-uniformity of the main system water supply has a strong effect on the water management in the tertiary units. Water shortage results in a lower cropping intensity and fallow land, no possibility to cultivate rice, and a possible trend towards salinization in the Seila area; (ii) the farmers in both research areas modify the official rotation schedule. These modifications create a high flexibility in the rotational units: The water supply for different crops and plots is adjusted almost to the minute.Glossary and abbreviations Bahar Gate-Tender - FaWMDI Fayoum Water Management and Drainage Improvement Project - fe Feddan (1 fe = 0.42 ha) - FID Fayoum Irrigation Department - Ganabiah Secondary or Sub-Secondary - IIP Irrigation Improvement Project - Kafr Satellite Village of a Larger Village with a Mayor - Mesqah Tertiary Canal - MSL Mean Sea Level at Alexandria - Nasbah Cluster of Fayoum Standard Weirs - Sheikh el Balad The Oldest of the Kafr (Elected Position)     

The reorganization of water users' associations in Mendoza,Argentina

This paper describes the changes introduced in water users' associations in the Province of Mendoza, Argentina. Before 1985 there were 709 water users' associations, known as Canal Inspection, each of them in charge of the administration of an area of about 300 ha, corresponding to tertiary and quaternary canals. At present there are 21 reorganized inspections administering an average area of 6 000 ha. The democratic system for the election of authorities is described and the method of budget preparation and control is discussed. The advantage of reorganization is illustrated through the economic assessment of the Reduccion Main Canal. This canal irrigates an area of 13 985 ha. The annual benefits due to reorganization are estimated to total A 32 800 (US$ 41 000), being 2.1 times the annual budget of the Inspection.     

Plant indicators of wheat and soybean crop water stress

Summary The onset of water stress within a crop is defined as the time at which the rate of water loss declines below that of a well watered crop in the same locality. The relation to the onset of water stress and soil water status of several readily measured plant parameters was investigated in crops of wheat and soybeans over three years. Evapotranspiration ET was monitored with weighing lysimeters. A noticeable decline in the rate of ET for both wheat and soybeans was detected once 20% to 30% of the total plant available water PAW remained in the 1 m deep lysimeter soil profile. Extension growth of wheat declined when PAW was 33% and 34% in two years of measurement. In soybeans, the decline in the rate of leaf extension coincided with the decline in the rate of ET. Midmorning measurement of exposed leaf water potential _L, covered leaf water potential _CL and covered plant leaf water potential _CP yielded similar results for both wheat and soybeans. Day-to-day variability was least in _CP and most in _L. Values of _CP, _L and _CL decreased rapidly with PAW < 30%. Daily values of leaf diffusive conductance were variable but there was a general decline in conductance with PAW < 30%. It is suggested that _CL may be the easiest and most reliable parameter to monitor as a means of detecting the onset of stress. The results indicated that PAW levels in the root zone of 50% for wheat and 30% for soybean probably do not affect extension growth or plant water status parameters and can thus be used as criteria for irrigation scheduling.Seconded from the Water Research Commission, Pretoria; present address: CSIRO, Division of Irrigation Research, Griffith, N SW 2680, Australia     

Evaluating the width of hydrological buffer zones between drained agricultural land and nature reserve areas

This paper presents the development of a practical tool to evaluate the required width of a hydrological buffer zone, in order to maintain the effects of subsurface drainage within reasonable limits. A simple mathematical formulation describing the lateral drawdown extent of a drained water table in unsteady flow conditions was used. Simulation results were worked out by introducing the concepts of protection level or threshold of tolerance for the vegetation of the protected area The latter are expressed in terms of an admissible drawdown combined with exceedance duration and/or frequency criteria. An application of this method to the protection of a peat bog system in the Swiss Jura region is presented.     

Procedure for predicting and designing moving sprinkler application patterns

Summary Water application pattern, WAP, is one of the most important factors that determine the instantaneous and the cumulative application rates of moving irrigation machines. The mathematical background of a procedure to predict and design the WAP of moving irrigation machines is introduced. It includes a mathematical analysis of the effect of pressure head, height and spacing between emitters on the WAP, and a nomograph that presents this analysis graphically and illustrates the design procedure of the application pattern of irrigation machines.Abbreviations P()a water application rate at a normalized radial distance from the emitter [m/s] - ka number of linear segments needed to represent the pattern - s/Ra normalized radial distance from the emitter - Ra wetted radius [m] - sa radial distance from the emitter [m] - n _j n _i/ha normalized water application rate at point - j, ha maximum water application rate [m/s] n _j water application rate at point j [m/s] - _j =m _j/Ra normalized radial distance of point j from emitter - m _ja radial distance of point - ja from emitter [m], CWAP - (x)a Cumulative Water Application Pattern: amount of water per unit area applied at a distance - xa from the travel path of the emitter [m³/m²] - xa distance from the travel path of the emitter [m] - T _xa time of application at a distance - xa from the travel path of the emitter [s] - va velocity of propagation of the machine [m/s] - k ₁a the outmost linear segment that its radial distance from the emitter - m _k1a is smaller than the distance of the travel path from the emitter - x, T _ja time at which the - j ^tha linear segment (ring) stops influencing the point located at a distance - xa from the emitter - ₁, ₂, ₃a dimensionless numbers derived by dimensional analysis - ua water jet velocity [m/s] - ga gravity acceleration [m/s²] - da nozzle diameter [m], v kinematic viscosity [m²/s] - Ha emitters height [m] - , a regression analysis coefficients - Paa Pattern fit coefficient for water application - F(r)a normalized desired water application pattern [1/m] - f(r)a normalized actual water application pattern [1/m] - La common distance on which - F(r) and f(r)a are defined [m], SP spacing interval between emitters [m] - DSa dimensionless spacing interval between emitters - DSa variation of dimensionless spacing interval - Paa variation of Pa coefficient - Pa pressure head [kPa]     

Photosynthesis,leaf conductance,and water relations of cowpea under saline conditions

Summary Cowpea (Vigna unguiculata L.), grown widely under both irrigated and dryland conditions, is well adapted to drought and high temperature and is moderately salt tolerant. Data on photosynthetic response and regulation of water relations in cowpea under salinity stress is lacking. Therefore, in conjunction with a field plot experiment to establish the leaching requirement of cowpea, measurements were made of carbon dioxide assimilation rates (A) by ¹⁴CO₂ uptake, leaf conductances to H₂O (g₁) by tritum uptake, and to CO₂ (g), and leaf total water potential (_t ¹) and osmotic potential ( ¹).Cowpeas, grown in field plots containing Pachappa fine sandy loam (mixed, thermic, Mollic Haploxeraff), were irrigated daily with saline water (1,350 mg 1^–1 total salt concentration) to achieve leaching fractions of 0.17, 0.13, 0.09, 0.07, and 0.02. Cowpea maintained high leaf water potentials, high rates of CO₂ assimilation and high leaf conductances under moderately saline conditions (high leaching). Values of _t ¹ and ¹ for high leaching were consistently 50 to 200 J kg^–1 higher than for low leaching throughout the day. Calculating ¹ at full leaf turgor eliminated diurnal variation in ¹. As leaching decreased, however, A, g₁, and g, decreased significantly. About 45% of the 1°C assimilated by the leaf was incorporated rapidly into ethanol insoluble compounds. The relationship between A and g₁ for cowpea was similar to that reported for other crops.Contribution from the US Salinity Laboratory, USDA-ARS, 4500 Glenwood Dr., Riverside, CA. 92501, USA     

Water management for irrigation in Antiquity (Urartu 850 to 600 B.C.)

The kingdom of Urartu existed in Eastern Anatolia from about 850 B.C. to 600 B.C. Historical references and archaeological evidence indicate the considerable artistic and technical skill of the Urartian people. The supply of the kingdoms capital Tupa/Rusahinili with drinking and irrigation water is an outstanding example of a well-planned and excellently built historical inter-basin water management project. The system has been in operation for more than 2000 years and still serves, at least partly, its original purpose.     

Relationships between crop temperature,grain yield,evapotranspiration and phenological development in two hybrids of moisture stressed sorghum

Summary Recent studies have shown that the grain yields of corn (Zea mays L.) and wheat (Triticum aestivum L.) are related to the degree of water stress they undergo. The purpose of the study reported here was to establish relationships between crop temperature and the grain yields, phenological development, evapotranspiration rates (ET) and leaf water potential ( _l ) of two hybrids of grain sorghum (Sorghum bicolor L. Moench) subjected to varying levels of plant water stress. The study was conducted at the University of Nebraska Sandhills Agricultural Laboratory in 1978 on a Typic Ustipsamment (Valentine fine sand) soil. The sorghum hybrids used were RS 626 and NB 505. Four irrigation treatments were applied in order to subject the crops to varying levels of water stress during each of three major growth stages. Soil moisture was monitored with a neutron probe. ET was estimated with the water balance technique. Crop temperature was measured with an IR thermometer and leaf water potential was measured with a Scholander pressure bomb. Grain yields were reduced by water stress occuring at anytime during the growing season. Yield reductions were largest when stress occurred during only the grainfill period and were least when stress occurred during the entire growing season. The percentage reduction in sorghum grain yield can be described by an index involving the seasonal accumulation of the daily mid-day temperature differences between well-watered and stressed crops ( TSD). As TSD values increased, ET decreased. However, the correlation of ET with TSD was relatively low (R² = 0.60) probably due to the limited amount of data available for analysis and inaccuracies in the soil water balance method used to estimate ET. The mid-day temperature of well-watered rows ranged between 18.0 and 32.8 °C with a mid-day temperature range of about 0.5 °C between the well-watered rows in various plots for several days following an irrigation. However, in certain instances, the mid-day temperature range increased to 1–2 °C for a few days before irrigation. This suggests that certain of the rows experienced water stress and should have been irrigated earlier. Yield data support that conclusion. Range in crop temperature within a field appeared to be a sensitive indicator of crop water stress in sorghum. No significant difference in the phenological development of sorghum resulted from water stress except in one NB 505 plot in which plants were stressed throughout the entire season. In that plot, the stressed plants lagged in development behind non-stressed plants by approximately ten days. The differences in mid-day leaf water potentials ( _l ) and crop temperatures (T) between stressed and non-stressed vegetation were examined. As T increased up to about 4 °C, _l , also increased. Beyond that point, _l decreased while T continued to increase. This behavior was attributed to stomatal closure which permitted an increase in _l of the stressed plants (hence reducing _l ) even as T continued to increase.Published as Paper No. 6551, Journal Series, Nebraska Agricultural Experiment Station. The work reported was conducted under Regional Research Project 11–33 and Nebraska Agricultural Experiment Station Project 11–50. The work upon which this publication is based was supported in part by funds provided by the Office of Water Research and Technology B-044-NEB, US Department of the Interior, Washington, DC, as authorized by the Water Research and Development Act of 1978. This article was sponsored in part by the Nebraska Water Resources Center, Institute of Agriculture and Natural Resources, University of Nebraska-LincolnResearch Assistant, Associate Professor, Research Assistant, and Associate Professor, University of Nebraska, Lincoln. Contents of this puplication do not necessarily reflect the views and policies of the Office of Water Research and Technology, US Dept. of the Interior, nor does mention of trade names or commercial products constitute their endorsement or recommendation for use by the United States Government     

Water-conserving wheat irrigation schedules based on climatic records

Summary Correlations between spring rainfall and grain yield were determined for four winter wheat cultivars (Triticum aestivum L. em. Thell. Triumph, Wichita, Concho, and Triumph 64), grown between 1950 and 1977 under dryland conditions at Stillwater, in the East Central region of Oklahoma, and at Goodwell, located in the drier, western part of the state.At Stillwater, all but one of the cultivars exhibited maximum positive correlations between rainfall and yield in the fourth week of March, when stem-extension occurs. Smaller positive correlations were observed in mid-April when flowering. Results at Goodwell were similar except that the correlations between rainfall and yield were lower and occurred earlier than at Stillwater and showed a less marked secondary peak at flowering. These results agree with those of experiments in which irrigation has been applied at different growth stages of wheat, and have shown that both stem-extension and flowering are critical stages of water requirement. As the results of this climatic study show that the peak correlations between rainfall and yield occur at these same two stages, it is suggested that long-term climatic data could be used to determine optimum timing for irrigation of wheat.Such an approach should save water and energy by limiting irrigation to those times when analysis of local records demonstrates the maximum positive correlation between rainfall and yield.     

Accounting for slow drainage and hysteresis in irrigation scheduling

Summary Soils continue to drain for several days following irrigation. Water budgeting predictions can be improved by accounting for this, provided hysteresis is recognized in the field capacity condition. Uncertainty in this condition was evident in experiments comparing predicted soil water depletions with neutron probe measurements. Comparisons were made for potato, lettuce and calabrese crops, irrigated by hose-reel machine. Best agreement between measurements and predictions was obtained when excess water storage above a lower field capacity condition was allowed for; corresponding to minimum root mean square errors of 3.2 mm–6.2 mm. These were comparable to the practical limits of uncertainty associated with field depletion measurements.