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1.
Summary A coupled soil-vegetation energy balance model which treats the canopy foliage as one layer and the soil surface as another layer was validated againt a set of field data and compared with a single-layer model of a vegetation canopy. The two-layer model was used to predict the effect of increases in soil surface temperature (T s ) due to the drying of the soil surface, on the vegetation temperature (T v ). In the absence of any change in stomatal resistance the impact of soil surface drying on the Crop Water Stress Index (CSWI) calculated from T v was predicted. Field data came from a wheat crop growing on a frequently irrigated plot (W) and a plot left un watered (D) until the soil water depletion reached 100 mm. Vegetation and soil surface temperatures were measured by infrared thermometers from tillering to physiological maturity, with meteorological variables recorded simultaneously. Stomatal resistances were measured with a diffusion porometer intensively over five days when the leaf area index was between 5 and 8. The T v predicted by the single-layer and the two-layer models accounted for 87% and 88% of the variance of measured values respectively, and both regression lines were close to the 11 relationship. Study of the effect of T s on the CWSI with the two-layer model indicated that the CWSI was sensitive to changes in T s . The overestimation of crop water stress calculated from the CWSI was predicted to be greater at low leaf area indices and high levels of stomatal resistance. The implications for this bias when using the CWSI for irrigation scheduling are discussed.List of Symbols C Sensible heat flux from the soil-vegetation system (W m–2) - c l shade Mean stomatal conductance of the shaded leaf area (m s–1) - c l sun Mean stomatal conductance of the sunlit leaf area (m s–1) - c max Maximum stomatal conductance (m s–1) - c 0 Minimum stomatal conductance (m s–1) - C p Specific heat at constant pressure (J kg–1 °C–1) - C s Sensible heat flux from the soil (W m–2) - C v Sensible heat flux from the vegetation (W m–2) - c v Bulk stomatal conductance of the vegetation (m s–1) - CWSI Crop Water Stress Index (dimensionless) - e a Vapor pressure at the reference height (kPa) - e b Vapor pressure at the virtual source/sink height of heat exchange (kPa) - e 0 * Saturated vapor pressure at T 0 (kPa) - e s Vapor pressure at the soil surface (kPa) - e v * Saturated vapor pressure at T v (kPa) - G Soil heat flux (Wm–2) - GLAI Green leaf area index (dimensionless) - GLAIshade Green shaded leaf area index (dimensionless) - GLAIsun Green sunlit leaf area index (dimensionless) - k Extinction coefficient for photosynthetically active radiation (dimensionless) - k 1 Damping exponent for Eq. A 5 (m2 W–1) - LAI Leaf area index (dimensionless) - LE Latent heat flux from the soil-vegetation system (W m–2) - LE s Latent heat flux from the soil (W m–2) - LE v Latent heat flux from the vegetation (W m–2) - p a Density of air (kg m–3) - PARa Photosynthetically active radiation above the canopy (W m–2) - PARu Photosynthetically active radiation under the canopy (W m–2) - r a Aerodynamic resistance (s m–1) - r b Heat exchange resistance between the vegetation and the adjacent air boundary layer (s m–1) - r c Bulk stomatal resistance of the vegetation (s m–1) - R n Net radiation above the canopy (W m–2) - R s Net radiation flux at the soil surface (W m–2) - r st Mean stomatal resistance of leaves in the canopy (s m–1) - R v Net radiation absorbed by the vegetation (W m–2) - r w Heat exchange resistance between the soil surface and the boundary layer (s m–1) - S Photosynthetically active radiation on the shaded leaves (W m–2) - S d Diffuse photosynthetically active radiation (W m –2) - S 0 Photosynthetically active radiation on a surface perpendicular to the beams (W m–2) - T a Air temperature at the reference height (°C) - T b Temperature at the virtual source/sink height of heat exchange (°C) - T 0 Aerodynamic temperature (°C) - T s Soil surface temperature (°C) - T v Vegetation temperature (°C) - w 0 Single scattering albedo (dimensionless) - Psychrometric constant (kPa °C) - 0 Cosine of solar zenith angle (dimensionless)  相似文献   

2.
Grapevines are extensively grown in the semiarid and arid regions, but little information is available on the variability of energy partitioning and resistance parameters for the vineyard. To address this question, an eddy covariance system was applied to measure energy balance over a vineyard in northwest China during 2005-2006. Result indicated that 2-year average Bowen ratio (β) of vineyard was 1.0, canopy resistance (rc) 289.3 s m−1, aerodynamic resistance (ra) 9.7 s m−1 and climatological resistance (ri) 117 s m−1. This implied that the annual energy was split almost equally between sensible heat and latent heat. Compared to the corresponding values in other ecosystems reported by Wilson et al. [Wilson, K.B., Baldocchi, D.D., Aubinet, M., Berbigier, P., Bernhofer, C., Dolman, H., Falge, E., Field, C., Goldstein, A., Granier, A., Grelle, A., Halldor, T., Hollinger, D., Katul, G., Law, B.E., Lindroth, A., Meyers, T., Moncrieff, J., Monson, R., Oechel, W., Tenhunen, J., Valentini, R., Verma, S., Vesala, T., Wofsy, S., 2002. Energy partitioning between latent and sensible heat flux during the warm season at FLUXNET sites. Water Resource Research 38, 1294-1305.], the vineyard had a higher β, rc and ri than deciduous forests, corn and soybean, and grassland. Such difference was mainly attributed to (1) serious water stress in 2005, which resulted in a greater rc up to 364.4 s m−1; (2) sparse canopy with row spacing of 2.9 m and plant spacing of 1.8 m; (3) warm-dry climate and high attitude (1581 m) along with higher ri and lower psychrometer (54 Pa K−1) in the arid region of northwest China. These characters of vineyard revealed varying process of energy partitioning and surface resistance, and provided a scientific basis in understanding and modeling water and energy balance for the vineyard in the semiarid and arid regions.  相似文献   

3.
Actual evapotranspiration (ETc) of three mature sweet orange orchards (cv. Salustiana and Washington Navel on sour orange), under border irrigation and typical cultural practices was measured by the water balance method during 1981 to 1984. Soil water content was measured at 7 to 10 day intervals using a neutron meter and soil sampling of the 0–10 cm surface layer. Zero flux plane was calculated by measurements with mercury tensiometers. Irrigation water in these and other 5 similar orchards was measured by broad crested weirs. Rainfall and other climatic data for calculation of reference evapotranspiration by FAO's methods (ETo) were collected in a nearby meteorological station. Average yearly ETc ranged from 750 to 660 mm and mean monthly maximum was 3.7 and 3.2 mm/day in July for Salustiana and W. Navel orchards, respectively.ETo estimates for the different methods used were highly correlated (r 20.94). Monthly crop coefficients (Kc) based on pan evaporation ranged from 0.5–0.6 in spring and summer to 0.8 in autumn and were about 10% higher than those for Penman or radiation methods. Average annual Kc for the three plots studied was 0.64, 0.61 and 0.51, respectively, and correlated well (r 2=0.99) with tree ground cover. Irrigation efficiency was about 50% for orchards with soils with less water holding capacity and more applied water per irrigation and 70–80% in orchards with deeper soils or with a higher water holding capacity. Increasing irrigation frequency and applying smaller amounts of water per irrigation with good uniformity can improve on-farm irrigation efficiency.  相似文献   

4.
The Bowen Ratio-Energy Balance (BREB) is an accurate method often used to measure the latent heat flux (λE) due to its simplicity and portability. However, its performance in advective areas is less clear and its accuracy may depend on the equality of eddy transfer coefficients for heat and water vapor. In this work, hourly measured λE of a reference crop (Festuca arundinacea Schreb.) using a BREB system was compared with lysimeter-measured λE under moderate to severe advective conditions. The lysimeter resolution for hourly records was 22.6 W m−2. The analysis of the eddy transfer coefficients was made using simultaneous measurements of fluxes and vertical gradients of temperature and humidity. To avoid computational problems when β→ −1, some hourly periods were discarded in the analysis. Rejected data amounted to 37% of the total, although the cumulative evapotranspiration (ET) during these hours did not exceed 13% of the total ET. The BREB method overestimated daily ET by an average of 5.5% and by 5.7% when only daylight hours were considered. Under stable atmospheric conditions the method was less accurate, with relative errors of 21% vs. 11% under unstable conditions. For daylight hours, accuracy was higher under unstable conditions (RMSE = 36.15 W m−2) than under stable conditions (RMSE = 50.20 W m−2), which had larger overestimations of ET (6.3 vs. 5.1%). The main source of error appears to come from insufficient fetch resulting in local advective conditions. Nevertheless, and from a purely practical perspective, under the advective conditions of these measurements the BREB technique provides accurate ET fluxes with limited errors.  相似文献   

5.
Summary A simulation model of water uptake by a crop was developed to facilitate synthesis of field and laboratory observations with existing knowledge, and to analyze and predict affects of management practices, such as tillage, on water uptake from a drying soil. Radial water flow resistance in soil Rs was estimated by the single root flow model. Leaf stomata closure was represented by an observed minimal leaf water potential. Flow resistances, per unit root length Rr and in the plant Rp, were assumed to be constant and were evaluated together with an effective root length factor Frl, in the course of simulating a ten week period of observed soil water depletion by a crop of oats. Rr, Rp, and Frl were found to have similar values to those reported in the literature. Potential transpiration and evaporation and their ratio were estimated by the methods of Van Bavel (1966) and Denmead (1973). Evaporation reduction due to soil drying was estimated empirically.Cessation of soil water depletion (attainment of a permanent wilting soil water content) in the 0 – 20 cm soil layer, during the last ten-day period, was explained to be the net result of soil water extraction by the roots and backflow of water from the roots into the soil. Simulated onset of crop stress (closure of stomata) was found to be characterized by: (a) a steady decrease in average soil water potential, at a rate of about 500 cm-water per cm-soil water depletion; (b) a tenfold increase in the average soil resistance to radial flow, to about the same magnitude as average radial flow resistance in the roots; and (c) soil water diffusivities in the 0 – 50 cm layer being about 6 cm2/day. Sensitivity analyses showed that the ratio of actual to potential cumulative transpiration RCT depended primarily on potential evapotranspiration, rainfall, the unsaturated-to-saturated hydraulic conductivity exponent and plant cover. RCT was affected similarly by changes in Rr and in Rs. Under the conditions tested, zero tillage may increase RCT significantly only if it increases deep rooting beyond the 50 cm depth.Joint contribution from the Georg-August University, Göttingen, FRG, and the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel, A.R.O. No. 207-E, 1980 Series  相似文献   

6.
Summary The Lewis-Milne (LM) equation has been widely applied for design of border irrigation systems. This equation is based on the concept of mass conservation while the momentum balance is replaced by the assumption of a constant surface water depth. Although this constant water depth depends on the inflow rate, slope and roughness of the infiltrating surface, no explicit relation has been derived for its estimation. Assuming negligible border slope, the present study theoretically treats the constant depth in the LM equation by utilizing the simple dam-break wave solution along with boundary layer theory. The wave front is analyzed separately from the rest of the advancing water by considering both friction and infiltration effects on the momentum balance. The resulting equations in their general form are too complicated for closed-form solutions. Solutions are therefore given for specialized cases and the mean depth of flow is presented as a function of the initial water depth at the inlet, the surface roughness and the rate of infiltration. The solution is calibrated and tested using experimental data.Abbreviations a (t) advance length - c mean depth in LM equation - c f friction factor - c h Chezy's friction coefficient - g acceleration due to gravity - h(x, t) water depth - h 0 water depth at the upstream end - i() rate of infiltration - f(x, t) discharge - q0 constant inflow discharge - S f energy loss gradient or frictional slope - S0 bed slope - t time - u(x, t) mean velocity along the water depth - x distance - Y() cumulative infiltration - (t) distance separating two flow regions - infiltration opportunity time  相似文献   

7.
Identification of drainage water allows assessing the effectiveness of water management. Passive capillary wick-type lysimeters (PCAPs) were used to monitor water flux leached below the root zone under an irrigated cropping system. Wireless lysimeters were developed for web-based real-time online monitoring of drainage water using a distributed wireless sensor network (WSN). Twelve PCAP sensing stations were installed across the field at 90 cm below the soil surface, and each station measured the amount of drainage water using two tipping buckets mounted in the lysimeter and continually monitored soil water contents using two soil moisture sensors installed above the lysimeter. A weather station was included in the WSN to measure micrometeorological field conditions. All in-field sensory data were periodically sampled and wirelessly transmitted to a base station that was bridged to a web server for broadcasting the data on the internet. Communication signals from the in-field sensing stations to the base station were successfully interfaced using low-cost Bluetooth wireless radio communication. Field experiments resulted in high correlation between estimated and actual drainage with r 2 = 0.95 and confirmed a reliable wireless communication throughout the growing season. A web-linked WSN system provided convenient remote online access to monitor drainage water flux and field conditions without the need for costly time-consuming supportive operations.  相似文献   

8.
Effective irrigation uniformity as related to root zone depth   总被引：1，自引：0，他引：1  
Summary In models used for relating the yield to irrigation uniformity it has been assumed that the spatial distribution of irrigation water, as measured at the soil surface, is indeed the water distribution at any depth throughout the root zone. In the present paper the distribution of infiltrated water within the soil bulk, as determined by an analytic solution of the two-dimensional unsaturated flow equation, did not conform to this assumption. A new alternative definition of irrigation uniformity is proposed under the assumption that water uptake by roots does not affect the flux distribution within the soil profile. In this analysis the spatial distribution of irrigation water flux at the soil surface, which is the upper boundary condition of the flow equation, is assumed to be a sine function. The solution to this problem indicates that there is a damping effect, which increases with soil depth, on the surface flux fluctuations. Furthermore, the actual irrigation uniformity at a given depth below the soil surface depends upon the initial uniformity at the surface and the distance between adjacent water sources. The closer the water sources are to each other, the shallower is the depth needed to damp the oscillations down to a certain level. This may explain why the actual uniformity of drip irrigation is high while the detailed distribution is very nonuniform and on the other hand, why the actual uniformity of sprinkler guns is low while the detailed actual distribution is close to uniform. Two uniformity coefficients are derived in this study: 1. A depth dependent coefficient which is made up of the damping factor that multiplies the flux fluctuations at the soil surface; 2. An effective uniformity coefficient, which is an average of the depth dependent coefficient over a part or the entire root zone. Different degrees of uniformity are expected when water is applied by different irrigation systems having similar uniformity coefficients at the soil surface, but dissimilar distances between the emitters. Assuming that crop yield depends to some extent on the uniformity of water depth actually available to the roots, the yields associated with such irrigation systems will probably also vary.  相似文献   

9.
Leaching requirement, the smallest steady-state leaching fraction which prevents any loss in crop yield, was determined for barley, cowpea, and celery in field plots at the U.S. Salinity Laboratory. Six replicated leaching-fraction treatments were irrigated many times each day with small quantities of water having an electrical conductivity of 2.3 dS/m. The crops were grown in succession between January 1979 and September 1981.The leaching requirement (Lr) was 0.10 for barley grain and 0.13 for barley forage. For cowpea seed, Lr was 0.16; 0.17 for cowpea forage. Lr for celery was 0.14. These experimentally determined values for barley and cowpea seed are higher by about 0.05 than those predicted by a leaching-requirement model based on an exponential crop water-uptake pattern. The experimental values for celery and cowpea forage are lower than predicted values by 0.06. These differences are not considered significant, however, when considered in terms of the small differences in water applications (about 25 mm) to cause these changes in Lr. Evapotranspiration during each crop's growing season coincident with Lr was 410, 630, and 460 mm for barley, cowpea, and celery, respectively.  相似文献   

10.
Camelina sativa (L.) Crantz is an oilseed crop touted as being suitable for production in the arid southwestern USA. However, because any significant development of the crop has been limited to cooler, rain-fed climate-areas, information and guidance for managing irrigated-camelina are lacking. This study measured the crop water use of a November-through-April camelina crop in Arizona using frequent measurements of soil water contents. The crop was grown under surface irrigation using five treatment levels of soil water depletion. The seed yields of treatments averaged 1,142 kg ha−1 (8.0% seed moisture) and were generally comparable with camelina yields reported in other parts of the USA. Varying total irrigation water amounts to treatments (295–330 mm) did not significantly affect yield, whereas total crop evapotranspiration (ETc) was increased for the most frequently irrigated treatment. However, total ETc for the camelina treatments (332–371 mm) was markedly less than that typically needed by grain and vegetable crops (600–655 mm), which are commonly grown during the same timeframe in Arizona. The camelina water-use data were used to develop crop coefficients based on days past planting, growing degree days, and canopy spectral reflectance. The crop coefficient curves, along with information presented on camelina soil water depletion and root zone water extraction characteristics will provide camelina growers in arid regions with practical tools for managing irrigations.  相似文献   

11.
The wind-tunnel technique is widely used to measure ammonia emissions from extensive surfaces. However, few studies have been made, within the tunnel, of the turbulent flow which drives the transfer processes above the experimental surface. Experiments have been carried out to determine the transfer characteristics within the tunnel, and the influence of the external wind on these characteristics. Two source types have been used, one with constant surface emission of CO2, and one with constant surface concentration of water vapour.Both water vapour emission rate and wind speed measurements show the building up of a boundary layer above the experimental surface, which was dramatically disturbed by the external wind in the first metre. Further downwind, the boundary layer stabilizes to a height of about 10 cm, below which the characteristics were equivalent to those of the atmospheric boundary layer, as shown by CO2and wind speed measurements. However, above the boundary layer, the wind speed in the tunnel was consistently lower than the external wind speed, showing that if the wind speed at 25 cm is used to drive the tunnel flow rate, the transfer rate in the end part of the tunnel would be consistently higher than outside.  相似文献   

12.
The effect of operating pressure heads on water application uniformity in microirrigation submain units was evaluated. Research results show that water application uniformity either increases or slightly decreases as operating pressure head increases in a range when the emission exponent x ≤ 0.5 in most cases. The water application uniformity decreases as operating pressure head increases in a range when the emission exponent x > 0.5. The relationship between operating pressure head and average emitter discharge in submain units can be considered as approximately linear for operating pressure heads in a small range (usually between the allowable minimum and maximum operating pressures of the submain units). These results help to estimate the average emitter discharge rate easily in a submain unit for an increased or decreased operating pressure head when one is attempting to manage emitter discharge dynamically according to the requirements of crop root growth for different periods. Generally, a microirrigation system designed to meet the desired uniformity of water application according to the allowable minimum operating pressure head would be better when x ≤ 0.5 because water application uniformity increases as operating pressure increases if emitter discharges are being managed dynamically. However, a microirrigation system designed to meet the required water application uniformity according to the allowable maximum operating pressure head would be better when x > 0.5 because, in general, water application uniformity increases as operating pressure decreases. Received: 29 November 1999  相似文献   

13.
Summary The use of canopy and air temperature differences to compute a crop water stress index (CWSI) for assessing plant water status was investigated using cotton crop canopies that either fully or partially covered the ground. The complete ground cover canopy condition was studied in a well watered moisture regime in a rainout shelter with measurements made on six Texas cotton race stocks. The partial ground cover canopy situation was investigated in a well watered moisture regime of a commercial cotton variety Paymaster 266 grown in the field. The slope of the nonstressed baseline of the CWSI for a cotton canopy with about 50% ground cover was approximately one-half that reported for full canopies. Values of CWSI calculated with theoretical and empirical procedures agreed more closely under a complete canopy condition than under a partial canopy situation. Values of aerodynamic resistance (r a ) and canopy resistance for well watered soil moisture conditions (r ep )were estimated in order to use the theoretical procedure of computing CWSI. Values of r a ranged from 10 to 15 sm–1 and r cp from 50 to 60 sm–1. Both the theoretical and empirical procedures showed much promise, but more information is needed to develop techniques for evaluating r a and r cp under differing canopy and environmental conditions.  相似文献   

14.
Water shortage is the major bottleneck that limits sustainable development of agriculture in north China. Crop physiological water-saving irrigation methods such as temporal (regulated deficit irrigation) and spatial (partial root zone irrigation) deficit irrigation have been tested with much improved crop water use efficiency (WUE) without significant yield reduction. Field experiments were conducted to investigate the effect of (1) spatial deficit irrigation on spring maize in arid Inland River Basin of northwest China during 1997–2000; (2) temporal deficit irrigation on winter wheat in semi-arid Haihe River Basin during 2003–2007 and (3) temporal deficit irrigation on winter wheat and summer maize in Yellow River Basin during 2006–2007. Results showed that alternate furrow irrigation (AFI) maintained similar photosynthetic rate (Pn) but reduced transpiration rate (Tr), and thus increased leaf WUE of maize. It also showed that the improved WUE might only be gained for AFI under less water amount per irrigation. The feasible irrigation cycle is 7d in the extremely arid condition in Inner River Basin of northwest China and less water amount with more irrigation frequency is better for both grain yield and WUE in semi-arid Haihe River Basin of north China. Field experiment in Yellow River Basin of north China also suggests that mild water deficit at early seedling stage is beneficial for grain yield and WUE of summer maize, and the deficit timing and severity should be modulated according to the drought tolerance of different crop varieties. The economical evapotranspiration for winter wheat in Haihe River Basin, summer maize in Yellow River Basin of north China and spring maize in Inland River Basin of northwest China are 420.0 mm, 432.5 mm and 450.0 mm respectively. Our study in the three regions in recent decade also showed that AFI should be a useful water-saving irrigation method for wide-spaced cereals in arid region, but mild water deficit in earlier stage might be a practical irrigation strategy for close-planting cereals. Application of such temporal and spatial deficit irrigation in field-grown crops has greater potential in saving water, maintaining economic yield and improving WUE.  相似文献   

15.
This paper deals with the prediction of the soil water retention h(S) and the soil unsaturated hydraulic conductivity K(S) functions of a clay-loam soil at a field scale (1 ha) where the variable S represents water saturation. The Van Genuchten model and the corresponding Mualem-Van Genuchten model were used to predict h(S) and K(S) functions respectively. The field data (tensiometric and neutron probe measurements) used in this study were provided by the soil water balance (four neutron sites, 0.35 to 1.55 m soil layer) of a soybean crop over a 78 days growing season. The advantages of the scaling approach for describing the field variability of the h(S) function were confirmed. The scaling approach accounted for 73% of the field variability of the soil matrix potential. A simple procedure was proposed in order to predict the K(S) function using scaling theory. This was done by simultaneously applying a ``zero flux method' and ``deep flux method' to compute the soil water balance and fit the saturated hydraulic conductivitiy (K sat), the only unknown parameter in K(S). Received: 15 November 1995  相似文献   

16.
The effect of irrigation dynamics and soil physical properties on the permissible rate of application was analyzed using the analytic solution of Richard’s equation for periodic flux type boundary conditions typical to high frequency irrigation. Dynamics of irrigation regime was defined by its instantaneous application rate, R 0, its frequency, and the duty cycle which is the ratio of irrigation duration and irrigation interval (period). Soil properties were saturation, hydraulic conductivity, diffusivity, and Gardner’s soil type coefficient. Fourier Transform was used to derive a closed form analytical expression for the maximal permissible value of R 0, which would not result in water logging and saturation at soil surface. The analytical expression ties the three irrigation parameters with the above three soil parameters together. Prevention of aeration stress by restricting the moisture content at the soil surface, to become less than the minimal air pore volume (drainable porosity), was also used as an upper constraint of moisture at the surface soil. The effect of irrigation frequency and duty cycle on the permissible R 0 values was analyzed and computed regarding three soil types: coarse sand, sand, and sandy loam. Under short duration periods of water application resulting from either small values of duty cycle or short irrigation periods (high frequency), or both, the soil surface would not become saturated even for very high R 0 values. The maximal application depth V max, depends on both the duty cycle and the frequency. For a given soil, V max remains essentially constant per irrigation periods of 1 h or less, typical to pulsed irrigation and independent of the duty cycle. For periods longer than 1 day for coarse soils, V max increases with the duty cycle while for sandy loam soils, the increase in V max becomes insignificant for duty cycles larger than 0.3. The computed values of maximal R 0 and V max based on the water logging concept are much higher than the ones used in irrigation design and should be considered as upper permissible limits only. The permissible values of R 0 and V max are much lower if the aeration stress concept is adopted and found in the range used in practice. This analysis may be useful for the design of precision irrigation for high frequency microdrip, high frequency trickle irrigation, as well as for trunk diameter measurement (TDM) irrigation methods.  相似文献   

17.
Crop water stress index relationships with crop productivity   总被引：1，自引：0，他引：1  
Summary Field experiments between 1983 and 1987 were used to study the effect of crop development on crop water stress index (CWSI) parameters and the relationship of CWSI with the yield of cotton and grain sorghum. The absolute slopes of nonstressed baselines (NSBL) generally increased until canopy cover reached 70% (Table 1). NSBL derived from data collected when canopy temperature exceeded 27.4 °C had greater absolute slopes and higher R 2-values than NSBL that included all diurnal measurements (Table 1). Average CWSI values of cotton and grain sorghum grown under varying soil water regimes were negatively correlated with yield. Grain sorghum yield was more sensitive to CWSI values than was cotton lint yield (Figs. 1 and 2). Multiyear data analysis indicated that yields from cotton that experienced a completely stressed condition during part of each day during the boll setting period would be 40% of those from completely nonstressed cotton (Fig. 3). Negative values of CWSI computed for cotton growing under non-water stressed conditions were associated with uncertainties in calculations of aerodynamic resistance (r aand in estimating canopy resistance at potential evapotranspiration (r cp).  相似文献   

18.
An analytic model to calculate evaporation from fetch-limited water bodies is described. By modifying the surface boundary condition to an analytic solution to the advection-diffusion equation for specific humidity in the air flow over a water body, we are able to solve for the entire specific humidity field q (x, z) from a single measurement of humidity, surface temperature, and wind speed. Comparisons of model predictions with measurements from Rushy Billabong, a small turbid lake, over a 146 day period show that on average the model underestimates evaporation rates by 12%. We believe that the evaporation shortfall is due to the downwind advection of heat within the billabong when the billabong is highly stratified in temperature. When the thermal stratification is weak, the advection of heat within the water column is less important and the model is an accurate predictor of evaporation.  相似文献   

19.
Canopy temperature, which may be estimated by infrared thermometry (IRT), can serve as an indicator of plant water status. [Idso et al., 1981a] and [Idso et al., 1986] proposed the nowadays much used concept of the crop water stress index, which relates observed canopy surface temperature (Ts) to maxima and minima temperature bounds. Jackson et al. (1981) defined those bounds on the basis of the energy balance. Those bounds vary with the meteorological situation. In this paper a chart is offered for general use with a fixed frame for the upper and lower bound. It relates canopy surface temperatures with r1(=1 + rc/ra)-values (rc the canopy resistance and ra the aerodynamic resistance) as a function of a specifically defined temperature sum (S). It links the curved lower bound with the straight upper bound by a bundle of r1-curves (the Ts-S-r1-chart). The lower bound can be expressed by an equation, which approximates the energy balance solution with high accuracy. The sensitivity of the upper bound is also discussed. A comparison was made between bounds following Jackson et al. (1981) and the proposed alternative method, which, however, is limited by the short data-set available for this paper.  相似文献   

20.
Laboratory experiments were conducted to investigate the effects of gravel mulch on evaporation with emphasis on resistance to water vapor transfer in a soil-mulch–atmosphere continuum. Based on measurements of evaporation rate using weighing soil columns with and without the surface mulch, the resistance to vapor flow by each of atmosphere, mulch and soil sections was determined. Comparison of experimental results of bare soil with mulched surface indicated that gravel mulch increases the resistance not only above the soil surface but also below the surface. The resistance of the mulch layer to water vapor flow above the soil surface increased exponentially with the thickness of the mulch layer, although so-called “aerodynamic resistance” (i.e. resistance above the mulch–atmosphere interface) slightly decreased compared to the bare soil because of large roughness of gravel mulch. The increase in the resistance below the soil surface, which is conventionally called “soil surface resistance”, is attributed to the development of a dry surface layer. Partial covering of the soil surface with gravel induced concentrated evaporation flux and increased depth of the drying front, even if the soil is relatively wet on spatial average. The nonhomogeneity in soil moisture movement may be reinforced when gravels are partially embedded into the soil.  相似文献   

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