Weighable Lysimeters Research Articles

Design and Operability of a Large Weighable Fen Lysimeter

In principle, conventional lysimeters are suitable for the investigation of vertical water and solute fluxes. Lateral fluxes in water-saturated fen sites are characterized by heterogeneities and abnormities due to anisotropic layering. But due to lack of adequate monitoring techniques, these fluxes have been insufficiently analyzed. The newly developed large weighable fen lysimeter (LWFL) overcomes the limitations of conventional lysimetry and enables the measurement of vertical and horizontal transport processes in undisturbed large volume soil monoliths. The LWFL has a volume of 6 m3 (4 m length, 1 m width and 1.5 m depth) and was tested by filling the lysimeter with an undisturbed fen monolith. A special extraction procedure for the horizontal sliding of the lysimeter vessel through the natural fen was developed. In front of the vessel a converted cutting tool assisted in carving the soil monolith out of the peat, both vertically and horizontally. Inlet and outlet of the LWFL was constructed to allow the adjustment of a wide range of hydraulic gradients to depict natural occurring lateral transport processes. The LWFL including the measurement techniques was tested successfully for 3 years. On the basis of these tests, we conclude that complex physical and biogeochemical research problems involving lateral flows can be tackled now with multiphase observations and measurements at high spatial and temporal resolution, transdisciplinary data evaluation and numerical modelling approaches.

Measurement of dew, fog, and rime with a high‐precision gravitation lysimeter

AbstractExact information about soil water flow is needed to quantify solute transfer within the unsaturated zone. Water flux densities are often measured indirectly, e.g., with water‐balance, water content–change, or tracer methods, and, therefore, often predicted with notable uncertainties. Over the last years, direct lysimetry methods have been increasingly used to study water and solute migration in soil profiles. A large weighable lysimeter is the best method to obtain reliable drainage data, but it requires relatively high investment and maintenance expenses. To reduce cost and improve comparability with undisturbed sites, a new technology to collect large monolithic soil columns with a surface area of 0.5–2 m2 and a depth of 1–3 m as well as a containerized polyethylene (PE‐HD) lysimeter station were developed. In addition, the station was fitted with a new high‐precision weighing technique. In this paper, the latter is demonstrated with data from a newly constructed gravitation lysimeter. Besides recording rainfall and seepage, its weighing precision makes it possible to register mass input by dew, fog, or rime. It also permits a very accurate calculation of actual evapotranspiration. Because this new type of lysimeter allows a very high temporal resolution, it is ideally suited to develop and test models for soil hydrologic processes.

A processing method for weighing lysimeter data and comparison to micrometeorological ETo predictions

Weighing lysimeters make direct measurements of water loss from a growing crop and the soil surface around a crop (evapotranspiration, ET), and thus, provide basic data to validate other ET prediction methods. The objective of this study was the improvement of the lysimeter data processing to eliminate bad data and minimize the effect of random variations in the measurements. Also, grass evapotranspiration rates (ETo) were compared with evapotranspiration rates predicted from data taken at a nearby weather station that is part of the California Irrigation Management Information System (CIMIS). Hourly data for a 2-year period from a grass lysimeter located at Five Points, California were screened to remove questionable data and data taken on rainy days (38% of all data). The hourly ETo rate was calculated for the remaining data from lysimeter mass measurements using a seven-point Savitsky–Golay (S–G) approximation of the derivative at the central point. The data were plotted against the CIMIS hourly ETo predictions using the Pruitt–Doorenbos (PD) model and the least-squares best-fit line was close to the 1:1 line indicating that CIMIS PD predictions were reasonably accurate for this time period. Comparing the S–G method with a two-point finite difference method, the S–G method reduced the root-mean-squared error between lysimeter ETo and CIMIS predictions from 0.112 for the two-point method to 0.074 mm h −1. Slopes of zero-intercept, best-fit lines to CIMIS PD versus lysimeter ETo for data representing each hour during the day exceeded unity from 9:00 a.m. to 4:00 p.m. but were less than unity at other times. An improved daytime result was obtained with the CIMIS Penman–Monteith (PM) model. The daytime soil heat flux calculated by the PM model is not included in the PD model and could account for this improvement. Correlation of hourly CIMIS and lysimeter ETo was poor during the night so CIMIS hourly ETo predictions at this site could be improved by characterization of factors that would improve prediction of ETo at night. Daily CIMIS ETo predictions were in good agreement with daily lysimeter ETo measurements for ETo < 6 mm d −1 but tended to be smaller when ETo > 6 mm d −1.

Ability of plant-based surface technology to improve urban water cycle and mesoclimate

The presented study proposes a technology that could be used in developing a stormwater management strategy. The research was conducted under the hypothesis that a plant-based surface system (PBSS) can increase the capacity of sealed urban surfaces, for example, along railway tracks, to reduce runoff volume, to increase evaporation and consequently to improve the mesoclimate. To test this statement, water retention capacity, runoff–precipitation ratio, runoff delay and evapotranspiration performance were determined for different time periods (rainwater events, daily, monthly, seasonally, yearly, and throughout the entire measuring period of 18 months) by means of a measurement strategy implemented in selected urban traffic areas of Berlin, Germany, that contained railway tracks. Additionally, a computation of the air humidity improvement of an urban test location based in measurements is presented. The main elements of the tested PBSS were a 2 cm thick vegetation mat and a 20 cm thick drainage layer. The measurement strategy for the hydrological characterization included the installation and use of weighable lysimeters, rain gauges and runoff volume counters. The runoff volume determined in some summer months for the PBSS technology amounted to 9% of precipitation. The evaporation performance in these months amounted to 100% of precipitation. These results were compared with the corresponding calculated values for an asphalt surface: runoff 70–80% of precipitation and evaporation performance of 20–30% of precipitation. The study provides evidence of the positive effect of this technology on the local water balance of the treated sealed areas.

Weighing Lysimeters for the Determination of Crop Water Requirements and Crop Coefficients

Weighing lysimeters are accurate instruments to measure crop evapotranspiration. Three weighing lysimeters, consisting of undisturbed 1.5- ×2.0-m surface area by 2.5-m depth cores of soil, were constructed and installed at the Texas Agricultural Experiment Station in Uvalde, Texas. Two lysimeters, each weighing approximately 14 Mg, were located beneath a linear irrigation sprinkler system and used in the field production of several crops commonly grown in the area. The third lysimeter was constructed and is used to measure reference ET from a well-watered, grass (ETos) located adjacent to the field lysimeters. Design construction, installation, engineering details and other considerations to ensure acceptable performance of the lysimeters are discussed. The lysimeter facility was developed to accurately assess crop water requirements of vegetables as well as other field crops grown in the Winter Garden region of Texas. Preliminary detection capability of the scale system is also reported.

Water use and crop coefficient of subsurface drip-irrigated lettuce in Central Arizona

A two year field study (1996/97 and 1997/98 growing seasons) was carried out at the Maricopa Agricultural Center (33º04'07" N; 111º57'18" W) of the University of Arizona, USA, to investigate the water use and to derive Kc's for subsurface drip-irrigated head lettuce grown in small weighable lysimeters. Measurement periods ranged from 480 to 1100 ºC-day (96/97) and from 439 to 1098 ºC-day (97/98). These intervals corresponded essentially to the second half of the crop cycle which amounted to a 1100 ºC-day, on average. The lysimeters were weighed periodically and the computation of the water balance revealed an average water use of 117 mm. Basal crop Kc was expressed as a function of cumulative growing degree days following a multiple linear regression procedure in which the data were fitted with a Fourier sine series model with up to six coefficients. Two-year Kc curves were obtained based on the Hargreaves, FAO Penman and FAO Penman-Monteith equations and compared to the AZSCHED (AriZona SCHEDuling) irrigation package. Predicted Kc peaked 0.88, 0.80 and 0.81 with the Hargreaves, FAO Penman, FAO Penman-Monteith equations, respectively, in the range of 1000 to 1050 ºC-day, in contrast to AZSCHED which predicted the peak Kc to be 1.01 at 1150 ºC-day.

AN EVAPOTRANSPIRATION RESEARCH FACILITY FOR SOIL-PLANT-ENVIRONMENT INTERACTIONS

The Soil-Plant-Environment Research (SPER) facility at USDA Agricultural Research Service at Bushland, Texas,opened in 1990 with 48 weighable lysimeters containing monolithic soil cores of three major soil groups and a rain shelter.Its purpose was to provide an environment for intensive plant water stress research and evaluate the impact of soil hydrauliccharacteristics on irrigation management strategies. The facility has now been upgraded with the addition of a fourth soiltype collected using static weights, the installation of 48 deck (platform) scales for the continuous measurement ofevapotranspiration (ET), and the use of a pressure-compensating drip system. The additional soil was selected to provide awider range of nutrient and water supplying capacity compared with the other three soils. The conversion to weighinglysimeters allows the collection of diurnal ET data needed for plant physiological studies and validation of ET measurementequipment and models. The drip system reduces labor, increases precision of application amount, and reduces disturbancearound the lysimeters. These improvements to the SPER facility will expand the type and quality of plant stress and irrigationmanagement research projects that can be performed.

Development and Testing of a Small Weighable Lysimeter System to Assess Water Use by Shallow-Rooted Crops

A mobile weighing system was designed in 1994 to weigh small lysimeters to measure water use by shallow-rooted crops. Using a reconditioned pesticide sprayer, a hoist, and a weighmeter, small lysimeters were weighed in the 1995, 1996, and 1997 growing seasons. The lysimeters were constructed of 4.8-mm hot-rolled steel and were 0.91 x 1.02 x 0.61 m in size with an internal area of 0.929 m 2 . The weight of a lysimeter containing only moist soil was 1.110.27 kg. At the beginning of each season of use, several tests were conducted on the weighing system to determine if it was sensitive enough to give adequate results of crop water use. In 1995, tests were conducted on linearity, repeatability, thermal shift, and creep errors. The values were ±0.062 kg, ±0.12 kg, -0.007 kg/ ° C, and -0.242 kg, respectively. The terminal linearity was 0.082 kg (1996) and 0.043 kg (1997). The measurement uncertainty decreased as the number of lifts increased. The results showed that the system was capable of producing adequate results for determining crop water use.

SOIL TEMPERATURE AND WATER EVAPORATION OF SMALL STEEL AND PLASTIC LYSIMETERS REPLACED DAILY

Soil water evaporation (E) measured by small, weighable lysimeters is affected by their size, construction materials, and replacement frequency. Steel and plastic are commonly used materials, but they have different thermal characteristics. Our objective was to investigate how wall material affects E and soil temperature of small lysimeters filled daily with undisturbed soil. Research was conducted during 5 days in August 1995 at Bushland, Texas, where the soil is a Pullman silty clay loam (fine, mixed, thermic Torrertic Paleustoll, 30% clay, 53% silt). Plastic and steel lysimeters 76 mm long, with inside diameters of 82 and 86 mm, respectively, were filled each morning by pressing them into undisturbed soil, and E was determined by mass change. Soil temperatures inside additional steel and plastic lysimeters were measured by thermocouples. No significant differences in E due to wall material were measured. For lysimeters of both wall materials, daily E ranged from 2 to 5 mm, daytime and nighttime E averaged 2.7 mm and 0.5 mm, respectively, and total cumulative E was 15.5 mm. Evaporation from small lysimeters was within 5% of E from a nearby large, precision, weighing lysimeter. Steel lysimeters were warmer at night near the surface, with significant differences from 0.5 to 0.9 °C, and warmer during the day at the bottom, with significant differences from 0.5 to 2.8 °C. Plastic lysimeters had greater vertical soil temperature differences than steel lysimeters. Significant differences inside plastic lysimeters ranged from about 2.0 to 3.5 °C greater than those of steel lysimeters during the daytime, and 0.5 to 1.0 °C greater during the nighttime. Measured temperature differences were consistent with greater thermal conductivity and enhanced heat transfer in steel sidewalls compared with plastic. Wall material affected temperature distribution, but not evaporation, of small lysimeters that were replaced daily.

Natural Geological Weighing Lysimeters: Calibration Tools for Satellite and Ground Surface Gravity Monitoring of Subsurface Water-Mass Change

Increased accuracy in measuring temporal variations in the Earth's gravity field allow inprinciple the use of gravity observations to deduce subsurface water-mass changes. This canbe with respect to a small area, or as a larger spatial average of water mass change usinggravity observations from low-altitude satellites, such as the forthcoming GRACE mission.At both scales, there is a need to validate gravity-based estimates against field recordings ofactual subsurface water-mass variations. In practice, this could prove difficult because thespatial integral of all water-storage change components can be subject to considerable fieldmeasurement error. An alternative approach to the validation process is proposed by whichsuitable geological formations are utilized as giant weighing devices to directly measure area-integratedwater-mass changes. The existence of such “natural geological weighing lysimeters”is demonstrated using observations from a replicated experimental site in New Zealand. Sitesof this type could be used to verify water-storage change estimates derived from sensitiveground surface gravity instrumentation. In addition, geological lysimeters could be used tomake local checks on the accuracy of any estimated regional water-mass time series, whichis proposed for satellite calibration. The land area “weighed” by a geological lysimeter increaseswith formation depth and it is speculated that recordings made at oil well depth may allowdirect monitoring of subsurface water mass changes at the regional scale.

A review of whole-plant water use studies in tree.

Weighing lysimeters, large-tree potometers, ventilated chambers, radioisotopes, stable isotopes and an array of heat balance/heat dissipation methods have been used to provide quantitative estimates of whole-tree water use. A survey of 52 studies conducted since 1970 indicated that rates of water use ranged from 10 kg day(-1) for trees in a 32-year-old plantation of Quercus petraea L. ex Liebl. in eastern France to 1,180 kg day(-1) for an overstory Euperua purpurea Bth. tree growing in the Amazonian rainforest. The studies included in this survey reported whole-tree estimates of water use for 67 species in over 35 genera. Almost 90% of the observations indicated maximum rates of daily water use between 10 and 200 kg day(-1) for trees that averaged 21 m in height. The thermal techniques that made many of these estimates possible have gained widespread acceptance, and energy-balance, heat dissipation and heat-pulse systems are now routinely used with leaf-level measurements to investigate the relative importance of stomatal and boundary layer conductances in controlling canopy transpiration, whole-tree hydraulic conductance, coordinated control of whole-plant water transport, movement of water to and from sapwood storage, and whole-plant vulnerability of water transport to xylem cavitation. Techniques for estimating whole-tree water use complement existing approaches to calculating catchment water balance and provide the forest hydrologist with another tool for managing water resources. Energy-balance, heat dissipation and heat-pulse methods can be used to compare transpiration in different parts of a watershed or between adjacent trees, or to assess the contribution of transpiration from overstory and understory trees. Such studies often require that rates of water use be extrapolated from individual trees to that of stands and plantations. The ultimate success of this extrapolation depends in part on whether data covering short time sequences can be applied to longer periods of time. We conclude that techniques for estimating whole-tree water use have provided valuable tools for conducting basic and applied research. Future studies that emphasize the use of these techniques by both tree physiologists and forest hydrologists should be encouraged.

Physiological reactions to imposed water deficit by Andropogon gayanus cv. Bisquamulatus and Cenchrus ciliaris cv. Biloela in a mixed fodder crop

Guinea grass (Andropogon gayanus Kunth cv. Bisquamulatus) and Buffel grass (Cenchrus ciliaris L. cv. Biloela) are grown as temporary fodder crops in the Sahelo-Sudanese region of Africa. Differences in their responses to drought conditions may be important in optimizing cattle feeding programmes in integrated farming systems. An experiment conducted in a wind tunnel equipped with a tank forming a weighable lysimeter compared net photosynthesis (A), stomatal conductance (gs) and leaf water potential (Ψ leaf) of both species in a mixed sward during three simulated environmental conditions: (1) rainy season, (2) drought period within the rainy season and (3) dry season.With a soil water reserve (SWR in % volume) of 8% during the drought period and the dry season, the Ψ leaf and A parameters were significantly lower (P&lt;0·001) when compared to those recorded with a SWR of 32% in the rainy season. Mean gs values were significantly different during each environmental condition (P&lt;0·001). No significant difference (P&gt;0·05) was found in gs and A between the two species during the rainy and the dry season or during a diurnal cycle, although a significant difference (P&lt;0·05) in Ψ leaf existed between the species during the dry season. During this season, Guinea grass was dormant while Buffel grass was growing slowly.When the drought period was imposed during the rainy season (SWR declined from 25 to 5%), A was reduced from 19·8 to 7·7 μmol m−2 s−1 for Guinea grass and from 16·8 to 3·7 for Buffel grass. Ψ leaf in both species dropped below −2 MPa while gs declined slowly from 0·70–0·80 to 0·55–0·60 cm s−1. A was significantly higher (P&lt;0·05) in Guinea grass during the last days of the drying cycle, but declined rapidly when drought was extended up to the wilting point. Two days after rewatering, all the parameters of both plants were restored to previous levels.It is concluded that Guinea grass was as drought-resistant as Buffel grass and that no competition for water existed between the species in the mixed sward. The study also gives relationships between the physiological parameters of both species and the soil water content during a drying cycle.

SPRINGTIME SOIL TEMPERATURES IN LYSIMETERS IN CENTRAL TEXAS

Weighing lysimeters are often used as a standard for evaporation measurements and are typically constructed with steel walls, a material with a thermal conductivity approximately 40 times that of soil. This highly conductive material can increase the rate of vertical energy transfer in the lysimeter and alter lysimeter soil temperatures. The objective of this study was to quantify effects of the vertical energy transfer by lysimeter walls on soil temperatures in lysimeters with surface areas of 0.175, 0.75, and 3.0 m2. Hourly average soil temperatures were measured at 0.1 and 0.4 m from 7 through 29 May 1985 in a field and at various positions in the lysimeters at the Blackland Research Center, Temple, Texas. The diurnal amplitude of hourly temperatures was smaller in the field than in lysimeters, was inversely related to lysimeter surface area, and decreased with distance away from the lysimeter wall. Mean daily temperatures in the lysimeters were up to 3°C greater than those in the field. Temperature differences were greater for smaller lysimeters. Diurnal temperature differences between the field and lysimeters were primarily caused by energy incident upon the exposed steel at the surface which was conducted into the soil by the wall. Temperature differences could impact early-season root and shoot growth and thus affect representativeness of lysimeter evaporation measurements. Lysimeters constructed wtih steel walls should be designed to minimize the exposed area of steel at the surface.

The accuracy of evaporation measurements from small lysimeters

Weighing lysimeters are generally considered the standard for evaporation ( E) measurements. They are usually installed as a single unit, so it is difficult to use them to determine treatment effects on E. The use of numerous, small lysimeters ( < 1 m 2 surface area) is one means of solving this problem, but small lysimeters may have design properties that could affect the accuracy of E measurements. The objective of this research was to evaluate the accuracy of sorghum and wheat E measurements from small lysimeters. Large, medium, and small lysimeters, with surface areas of 3.0, 0.75, and 0.18 m 2, respectively, were used. The single large lysimeter provided the standard to which seasonal and 5 to 29 day E values from four medium and four small lysimeters were compared. Seasonal sorghum E totals for the medium and small lysimeters were significantly different from the large lysimeter value ( α = 0.05, two-tailed), presumably because leaf area index (LAI) differed among lysimeters. When sorghum E values were adjusted to account for LAI differences, totals were not significantly different. When the means of daily E values from the medium or small lysimeters, calculated from the 5 to 29 day periods, were compared, using linear regression, to the value from the large lysimeter the slope was significantly different from 1.0. Adjusted for LAI differences, slopes were not significantly different from 1.0. For wheat, neither the mean seasonal E of the medium or small lysimeters was significantly different from the large lysimeter value, nor were the slopes different from 1.0. No consistent effect of lysimeter size on accuracy was found for either crop. Properly used, small lysimeters provide a technique for seasonal E measurements. Based upon the results from this study and previous work, statistical differences between treatments in seasonal E (using four small lysimeters, area <1 m 2) would not be demonstrable if the difference was less than 52 mm.

Sources of Error in Hydraulic Weighing Lysimeter Measurements

ABSTRACT HYDRAULIC weighing lysimeters are widely used for determining rates of evapotranspiration of agricultural crops. Some lysimeter systems have been modified to reduce temperature errors by placing the indicating manometer below ground level where temperature fluctuations are minimized or by using dummy mechanisms to allow for temperature-induced error correction. This paper contains the results of a laboratory study using a prototype size lysimeter to study accuracy and sources of error that might be caused by variable pillow contact area, temperature changes in various components of the indicating mechanism, and rate of lysimeter loading. The effect of degree of inflation of the hydraulic pillows on bearing plate contact area is examined in detail and found to have a significant effect on lysimeter calibration. Hysteresis in the indicating mechanism was not significant. The effects of rapid changes in lysimeter tank mass relative to gradual mass changes are evaluated and found not to significantly affect lysimeter response. The paper concludes with a discussion of the implications of the laboratory findings on the accurate collection of evapotranspiration data under field conditions..

Precise measurement system of evaporation from the land surface and a case study of observed results.

In order to investigate air-earth interaction, a systematic measurement system near the surface was developed and installed in the observation field after careful examination of instruments and data processing. Some aerodynamic approaches are provided for the determination of the vertical turbulent fluxes of heat and water vapor. Actual evaporation from the land surface is obtained from a weighable lysimeter of high sensitivity. The gradient method among the aerodynamic approaches provides a good estimate of evaporation. The relationships between evaporation and soil water content are investigated in detail by means of the lysimeter and electrical capacitance meters placed in the soil container of the lysimeter. The data show that the soil water content in the skin layer changes appreciably in a day and that its variation corresponds closely to the unevenness of the evaporation.

Dewfall and Evapotranspiration Determination during Day- and Nighttime on an Irrigated Lawn

Dewfall and evapotranspiration have been measured with a weighable lysimeter and computed through the energy balance equation, during day and nighttime; the Kazanskiy-Monin nomograph has been used for parameterizing the air sensible heat flux. The experimental data have been taken on a flat lawn, starting after an irrigation. The results show that the values of the computed mean net water flux are always significant, that is, their signs are always unambiguous. There is a general good agreement between derived and measured water fluxes, and a good coincidence between derived and computed dew onset, dew ending and dew duration.

Measuring evapotranspiration by hydrological methods

The measurement of evapotranspiration using the methods of micrometeorology requires a sophisticated application of advanced technology. The simpler alternatives, though they are not always less costly, are discussed, namely (1) precipitation-minus-stream discharge, (2) estimates of the changes of the water storages, either in the water table aquifer or in the unsaturated zone of the soil, (3) lysimeters, and (4) material balance methods. The accuracies attainable by these methods vary. The most precise estimate can be got from a well-designed, weighable lysimeter, but the representativeness of the exposure is always a worry for the experimenter. The least precise estimate, on a month-to-month basis, is afforded by the precipitation-minusstream discharge method, but this method can give a good estimate of annual total evapotranspiration. The range of evapotranspiration rates probably spans 0.7 mm d −1 in mid-winter, on occasions, to 5 mm d −1 in late spring if the season is a wet one. For reasonable definition of these rates on a daily basis, we are looking for a method that is accurate to ±0.2 mm d −1. Only the weighing lysimeter can achieve this. The other methods variously provide mean rates for periods of five days or longer.

The short-term fate of urea applied to barley in a humid climate. I. Experiments

The fate of 15N labelled urea and potassium bromide applied to a mole-tile drained silt loam soil, sown to barley, was investigated using microplots and small weighable lysimeters. Two irrigation treatments, corresponding to normal and high rainfall conditions, were imposed on the lysimeters. After 35 days approximately 90% of the applied nitrogen (N) was recovered from the lysimeters, in the soil, plants and leachate, indicating gaseous losses were not large. Approximately 50% of the urea N was hydrolysed within 3 days of application, and a similar percentage was present as organic N in the soil after 20 days. Six per cent of the fertilizer N was leached from the normal lysimeters and 14% from the wetter lysimeters. In contrast, 76% of the applied bromide was leached from the wetter lysimeters. Plant uptake into shoots and roots of fertilizer N was 32% and 22% of that applied in the normal and wetter lysimeters respectively, leaching losses being largely at the expense of plant uptake. Native soil N was also measured. It is suggested that process-oriented studies of the kind described can assist in the interpretation and extrapolation of results from conventional fertilizer, trials, particularly when used to develop simple mechanistic models.

Transpiration and leaf water potentials of wheat in relation to changing soil water potential

Changes in the transpiration rate of wheat in drying soils were followed in experiments in which plants were grown in two small weighable lysimeters in a glasshouse. Hourly measurements of soil water potential (Ψs) were made at three depths in each lysimeter. The water potential of flag leaves was measured with a pressure chamber, and stomatal resistance with a pressure drop porometer. Data on root densities and distribution were also obtained. Transpiration rates fell below estimated potential levels when the average value of Ψs in the root zone was reduced to –1 to –5 bars, depending on soil storage, root distribution and potential transpiration rate. From this point Ψs fell rapidly in the surface layers, more slowly at depth. It was found that accurate calculations of daily water uptake could be made from changes in soil water content. The minimum value of leaf water potential (�1 )attained each day declined progressively through the drying cycle, but there was evidence that stomatal resistance (rs) is not uniquely related to Ψ1; initial stomatal closure occurred at Ψ1, values which decreased from –11 to –25 bars as drying progressed. This adaptive mechanism is related to changes in osmotic potential of the leaves. Whole plant resistances (Rp), derived from leaf water potentials and fluxes through individual stems, increased as stem populations increased. In the high population lysimeter Rp decreased from 300 to 100 bar sec mm-3 as canopy transpiration rates increased from 1.5 to 4.5 x 10-4 mm sec-1. In the low population lysimeter Rp decreased from 70 to 30 bar sec mm-3 as transpiration increased from about 2.2 to 4.5 x 10-4 mm sec-1. The higher resistances appear to confer significant advantages in terms of water conservation and adaptation to drought.