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1.
This study was performed to test three methods based on the FAO-56 “dual” crop coefficient approach to estimate actual evapotranspiration (AET) for winter wheat under different irrigation treatments in the semi-arid region of Tensift Al Haouz, Marrakech (center of Morocco). The three methods differ in the calculation of the basal crop coefficient (Kcb) and the fraction of soil surface covered by vegetation (fc). The first approach strictly follows the FAO-56 procedure, with Kcb given in the FAO-56 tables and fc calculated from Kcb (No-Calibration method). The second method uses local Kcb and fc values estimated from field measurements (Local-Calibration method) and the last approach uses a remotely-sensed vegetation index to estimate Kcb and fc (NDVI-Calibration method). The analysis was performed on three fields using actual (AET) measured by Eddy Correlation systems. It was shown that the Local-Calibration approach gave best results. Accurate estimates of Kcb and fc were necessary for FAO-56 “dual” crop coefficient application. The locally derived Kcb for winter wheat taken at initial, mid-season, and maturity crop growth were 0.15, 0.90 and 0.23, respectively. The Kcb value at the mid-season stage was found to be considerably less than that suggested by the FAO-56.  相似文献   

2.
Soil infiltration problems occur as a result of alternating irrigation with saline-sodic waters and monsoon rainfall. Hydraulic conductivity (K) and related soil properties of a non-calcareous (CaCO3 0.8%) and a calcareous soil (25.7%) having similar textural constituents were monitored. The soils were subjected to six consecutive cycles of irrigation with saline waters (SW) of sodium adsorption ratio (SAR), 10, 20 or 30 (mmol/l)1/2, but of similar electrolyte concentration (EC; 80 mEq/l), and each followed by simulated rain water (SRW) (electrical conductivity <0.02 dS/m). Results are presented in terms of relative K i.e. K r=K sw/K tw where K tw is steady state K measured separately under application with tap water (ECw 0.54 dS/m, SAR 0.9). For irrigation with SW alone, K r values were reduced to 0.95, 0.79 and 0.70 at SAR of 10, 20 and 30, respectively, in non-calcareous soil. The corresponding values of 0.95, 0.87 and 0.79 were slightly higher in calcareous soil. Severe reductions in K r were observed in both the soils when subjected to alternate use of SW and SRW (K r=0.22, 0.03 and 0.02 in non-calcareous, and 0.57, 0.17 and 0.07 in calcareous soil). About half of the reductions in K r were reversible when SW was subsequently applied. Depth distributions of salinity, pH, dispersible clay and hydraulic head indicate that disaggregation and dispersion of surface soil was the cause of reduced K with SRW, whereas “washed in” sub-soil became restrictive and controlled the K values with SW under alternations of SW and SRW. Salt release (<1 mEq/l) was insufficient to avoid dispersion and sustain K even in the calcareous soil. For evaluating the infiltration hazard of saline-sodic water, measurements of stabilized K values after consecutive cycles of SW and SRW should serve as a better diagnostic criteria under monsoonal climates than threshold EC–SAR combinations. Received: 8 June 1998  相似文献   

3.
The field-saturated hydraulic conductivity (Kfs) finds use in many agricultural, environmental and geotechnical investigations and designs. It is commonly derived from falling-head tests in a well, but its derivation is sometimes based on assumptions that may not always hold. The aim of this study was to develop graphs for falling head percolation tests for 340 and 60 mm diameter wells—centred in two separate cylindrical soil tanks—for use in deriving Kfs of a fine sand placed in the tanks. The graphs were obtained from results from the finite element program SEEP/W that was used to model percolation from the tank wells into sand, fine sand and silt of known Kfs; the soils were all initially field-saturated, and the tests were carried out under (i) field-saturated (by spray irrigation) and (ii) draining conditions. For the modelled field-saturated tests on the three soils, the graphs of fall of head in the well, ΔHw, against the parameter Kfs × Δt, where Δt is the time taken for the particular fall ΔHw to occur, were coincident. This indicated that for a given measured fall, ΔHw, for a particular head of water, Hw, in a well, Kfs could be calculated by dividing the parameter Kfs × Δt, by the observed time, Δt, for the fall, ΔHw, to occur. For the modelled tests on the three draining soils, there was another inverse relationship between ΔHw and the parameter Kfs × Δt for small falls in well heads after start-up. There was good agreement between the Kfs value of a fine sand measured in a cylindrical constant-head laboratory permeameter and that obtained using the graphs and the experimental results from both the field-saturated fine sand, and the same fine sand in draining mode at small falls of head in the well. This suggests that falling head tests on non-irrigated soils—initially field-saturated—could be used to determine Kfs from early time falls in well heads. Since the time parameter greatly increases with increasing well size and decreasing pressure heads in the percolation well, it is quicker to carry out falling-head percolation tests in smaller test wells and at higher heads in the well. The procedure employed for obtaining Kfs for the soils within the Kfs range examined could be extended to the field.  相似文献   

4.
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   

5.
Water transmission characteristics under saturated and unsaturated conditions were studied in a sandy loam soil with (F1) and without (F0) long-term farmyard manure (FYM) treatments, in relation to sodium adsorption ratios (SAR) and electrolyte concentrations of water. The effect of FYM and ratios of Ca2+ : Mg2+ in water at a given SAR on sodication of the soil was also studied.Saturated hydraulic conductivity (k) and weighted mean diffusivity (D?) were slightly higher for F1 than for F0, whereas sodication indices like Gapon constant (KG), Krishnamoorthy-Davis-Overstreet constant (KKDO) and Vanselow constant (KV) were slightly smaller. The k and D? decreased with an increase of SAR and decrease of electrolyte concentration, the effect of SAR being more pronounced. There was proportionately a sharper decrease in the k and D? values at SAR 10 with total electrolyte concentrations of 10–40 meq 1?1. However, with a total electrolyte concentration of 80 meq 1?1, there was a smaller drop at SAR 10.A small difference in the build-up of exchangeable sodium percentage (ESP) in F1 and F0 treatments at a given SAR suggests that, apart from slightly improving water transmission parameters, the use of FYM also reduces the sodication hazard in a soil irrigated with sodic waters. An increase in the Ca2+ : Mg2+ ratio from 25:75 to 75:25 slightly decreased the values of KG, KKDO and KV, thus indicating somewhat more preference for Ca2+ to Mg2+ at a given SAR, which was more so in F1 soil. This fact could also be expressed in terms of a slight shift of thermodynamic exchange constant (K) and standard free energy change of the exchange reaction (ΔG0r). The presence of some unidentified Na+ releasing minerals in the soils studied was observed and correction for exchangeable Na+ determination applied.  相似文献   

6.
Accurate estimates of peanut (Arachis hypogaea L.) water requirements are needed for water conservation. The objective of this study was to evaluate the FAO-56 crop coefficients for peanut grown under various levels of water stress in a humid climate. Two experiments were conducted in three automated rainout shelters located at the University of Georgia Griffin Campus in Griffin, Georgia, USA in 2006 and 2007. Irrigation was applied when the modeled soil water content in the effective root zone dropped below a specific threshold of the available water content (AWC). The irrigation treatments corresponded to irrigation thresholds (IT) of 40, 60 and 90% of AWC. The soil water balance was used to compute observed evapotranspiration (ET cm) from measured soil water content at six different soil depths. The length of the four developmental stages was different than the values listed in FAO-56. The 2-year average absolute relative error of K cini was 8, 19 and 6% for 40, 60 and 90% IT, respectively. For the 90% IT, the FAO-56 K cmid and K cend were almost identical to the 2-year averages of the observed K cmid and K cend, respectively. The findings of this study confirmed that the FAO-56 procedure was reasonably accurate for estimating peanut ET under water stress in a humid climate.  相似文献   

7.
Soil water retention curves (WRCs) are required to predict the availability of water to plants and the movement of water through the soil. Soil kneading by hand is a type of extreme structure disturbance which is comparable to mechanical breakdown of soil structure by cattle trampling, tillage and wheeling of heavy machinery, especially when the soil is wet. The objective of this study was to explore how kneading affects soil water retention characteristic for a set of contrasting soils from southern Spain, and identify the soil properties that control the differences in behaviour. WRCs were measured at six soil-water matric potentials in the range of 10–1500 kPa on both undisturbed soil clods and kneaded soil samples in order to quantify differences in water retention characteristics due to sample pre-treatment. Results evidenced that only WRCs of sandy loam soils are not affected by soil pre-treatment; for the rest of the soils kneading increased water retention at low or high potentials or both. Particle size parameters are, for the most part, clearly related to the behaviour observed, but some other explaining variables linked to aggregation and dispersion state of phyllosilicates in the clay and silt fractions could also be involved.  相似文献   

8.
Camelina sativa (L.) Crantz is a promising, biodiesel-producing oilseed that could potentially be implemented as a low-input alternative crop for production in the arid southwestern USA. However, little is known about camelina’s water use, irrigation management, and agronomic characteristics in this arid environment. Camelina experiments were conducted for 2 years (January to May in 2008 and 2010) in Maricopa, Arizona, to evaluate the effectiveness of previously developed heat unit and remote sensing basal crop coefficient (K cb ) methods for predicting camelina crop evapotranspiration (ET) and irrigation scheduling. Besides K cb methods, additional treatment factors included two different irrigation scheduling soil water depletion (SWD) levels (45 and 65 %) and two levels of seasonal N applications within a randomized complete block design with 4 blocks. Soil water content measurements taken in all treatment plots and applied in soil water balance calculations were used to evaluate the predicted ET. The heat-unit K cb method was updated and validated during the second experiment to predict ET to within 12–13 % of the ET calculated by the soil water balance. The remote sensing K cb method predicted ET within 7–10 % of the soil water balance. Seasonal ET from the soil water balance was significantly greater for the remote sensing than heat-unit K cb method and significantly greater for the 45 than 65 % SWD level. However, final seed yield means, which varied from 1,500 to 1,640 kg ha?1 for treatments, were not significantly different between treatments or years. Seed oil contents averaged 45 % in both years. Seed yield was found to be linearly related to seasonal ET with maximum yield occurring at about 470–490 mm of seasonal ET. Differences in camelina seed yields due to seasonal N applications (69–144 kg N ha?1 over the 2 years) were not significant. Further investigations are needed to characterize camelina yield response over a wider range of irrigation and N inputs.  相似文献   

9.
Direct measurement of soil saturated hydraulic conductivity (Ks) is time-consuming and therefore costly. The ROSETTA pedotransfer function model is able to estimate Ks from soil textural data, bulk density and one or two water retention points. This study evaluated the feasibility of running the DRAINMOD field-scale hydrological model with Ks input produced using ROSETTA. A hierarchical approach was adopted to estimate Ks using ROSETTA, with four limited-more extended sets of soil information used as inputs: USDA textural class (H1); texture (H2); texture and bulk density (H3); texture, bulk density, water retention at −33 kPa (θ33 kPa) and −1500 kPa (θ1500 kPa) (H4). ROSETTA-estimated Ks values from these four groups (H1-H4) were used in DRAINMOD to simulate drain outflows during a 4-year period from a conventional drainage plot (CD) and two controlled drainage plots (CWT1 and CWT2) located in south-east Sweden. The DRAINMOD results using ROSETTA-estimated Ks values were compared with observed values and with model results using laboratory-measured Ks values (H0). Deviations in simulated drainage outflow (D), infiltration (F) and evapotranspiration (ET) resulting from the use of ROSETTA-estimated rather than laboratory-measured Ks values were evaluated. During the study period, statistical comparisons showed good agreement on a monthly basis between observed and DRAINMOD-simulated drainage rates using five soil datasets (H0, H1, H2, H3 and H4). The monthly mean absolute error (MAE) ranged from 0.57 to 0.82 cm for CD, 0.38 to 0.41 cm for CWT1, and 0.15 to 0.22 cm for CWT2. On a monthly basis, the modified coefficient efficiency (E′) values were in the range of 0.62 to 0.74 for CD, 0.72 to 0.74 for CWT1, and 0.79 to 0.86 for CWT2. The modified index of agreement (d′) for monthly predictions ranged from 0.80 to 0.86 cm for CD, 0.87 to 0.88 cm for CWT1, and 0.89 to 0.93 cm for CWT2. The absolute values of the percent-normalised error (NE) on an overall basis when using ROSETTA-estimated rather than laboratory-measured Ks values were less than 3% in E, less than 1% in F, and less than 15% in D. The results suggest that ROSETTA-estimated Ks values can be used in DRAINMOD to simulate drainage outflows as accurately as laboratory-measured Ks values (H0) in coarse-textured soils.  相似文献   

10.
Use of literature crop coefficient (K c) values for quantifying evapotranspiration (ETc) under non-standard conditions such as plastic mulch, shallow water table, and sub-tropical conditions can lead to inaccurate ETc estimates. A 5-year experiment was conducted for fall crop growing seasons in south Florida to quantity bi-weekly ETc and K c for bell pepper grown under shallow water table and plastic mulch environments using large drainage lysimeters. The ETc values varied from 205 to 320 mm with a seasonal average of 267 mm. Average K c values for bell pepper for development, mid-season, and late stages were 1.05, 1.21, and 1.28, respectively. Higher than literature initial K c values were due to rainfall and use of sub-irrigation system to maintain artificially high water table which results in high soil moisture in the bare soil area—such high moisture results in high evaporation. The K c values from this study were statistically higher than literature values. Use of literature K c values resulted in underestimating ETc by 27–37%. The K c values would provide improved estimates of sub-irrigated pepper ETc in subtropical Florida and elsewhere with similar environment.  相似文献   

11.
Consumptive water use and crop coefficients of irrigated sunflower   总被引：2，自引：1，他引：1  
In semi-arid environments, the use of irrigation is necessary for sunflower production to reach its maximum potential. The aim of this study was to quantify the consumptive water use and crop coefficients of irrigated sunflower (Helianthus annuus L.) without soil water limitations during two growing seasons. The experimental work was conducted in the lysimeter facilities located in Albacete (Central Spain). A weighing lysimeter with an overall resolution of 250 g was used to measure the daily sunflower evapotranspiration throughout the growing season under sprinkler irrigation. The lysimeter container was 2.3 m × 2.7 m × 1.7 m deep, with an approximate total weight of 14.5 Mg. Daily ET c values were calculated as the difference between lysimeter mass losses and lysimeter mass gains divided by the lysimeter area. In the lysimeter, sprinkler irrigation was applied to replace cumulative ET c, thus maintaining non-limiting soil water conditions. Seasonal lysimeter ET c was 619 mm in 2009 and 576 mm in 2011. The higher ET c value in 2009 was due to earlier planting and a longer growing season with the maximum cover coinciding with the maximum ET o period. For the two study years, maximum average K c values reached values of approximately 1.10 and 1.20, respectively, during mid-season stage and coincided with maximum ground cover values of 75 and 88 %, respectively. The dual crop coefficient approach was used to separate crop transpiration (K cb) from soil evaporation (K e). As the crop canopy expanded, K cb values increased while the K e values decreased. The seasonal evaporation component was estimated to be about 25 % of ET c. Linear relationships were found between the lysimeter K cb and the canopy ground cover (f c) for the each season, and a single relationship that related K cb to growing degree-days was established allowing extrapolation of our results to other environments.  相似文献   

12.
Field water supply (FWS) combines the three sources of water used by a crop for evapotranspiration (ET), and consists of available soil water at planting (ASWP), rainfall, and irrigation. Examining the grain yield and FWS relationship (Yg:FWS) may provide insight into the reported variability in crop water production functions such as water productivity (WP) and irrigation water productivity (IWP). Since water is most productive when entirely consumed in ET, diversion of FWS into non-ET losses such as drainage and excessive soil water evaporation results in declines in WP and IWP. The objective of this experiment was to examine the Yg:FWS and Yg:ET relationships of grain sorghum grown under a range of irrigation treatments (0, 25, 50, and 100% replacement of ET), beginning soil water contents, evaporative demands, in the Amarillo, Pullman, and Ulysses soils of the Great Plains. The purpose was to determine the amount of FWS beyond which declines in WP and IWP began to occur due to non-ET losses as indicated by a change in the slope and intercept of the Yg:FWS and Yg:ET relationships. Large amounts of non-ET irrigation application losses occurred in the finer-textured soils in the T-100 irrigation treatment. In both years, the T-100 irrigation application amounts and ASWP resulted in a FWS ranging from 750 to 870 mm which exceeded the maximum ET requirement of 530-630 mm and which reduced WP and IWP. Piecewise regression analysis of the Yg:FWS and Yg:ET relationships for the crops in the Pullman and Ulysses soils identified the knot point, or change in slope and intercept, in the FWS where both WP and IWP tended to be optimized. This was about 500 mm in both soils, and involved the utilization of about 250 mm in ASWP, irrigation applications averaging about 250 mm, and about 60-130 mm remaining in the soil at harvest. For the coarser-textured Amarillo soil, the yield response to increasing FWS was linear, because non-ET application losses such as drainage gradually increased with the irrigation application amount. The linear Yg response in the sandy Amarillo soil and the piecewise Yg responses in the clay and silt loams of the Pullman and Ulysses soils to FWS also reflected the difference in water-holding capacities of the soils that affected the amount of available water as irrigation increased. Irrigating without considering FWS resulted in non-ET irrigation application losses and declines in WP and IWP.  相似文献   

13.
Field-saturated and unsaturated finite element analyses (FEA) of constant-head well percolation tests at three well pressure heads in two large tanks with three soils of widely differing field-saturated hydraulic conductivities (Kfs) were carried out. The results of these analyses were compared with those from the Glover classical approach. The A values in the Glover-type formula (A = Kfs/Qs where A was a model parameter and Qs was the FEA steady-state infiltration rate into the soil) were shown to be independent of soil type for a particular pressure head and soil condition. Constant-head well percolation tests were carried out in two laboratory sand tanks with the same dimensional arrangement used in the finite element analyses. Field saturation was maintained by spray irrigation, and in the unsaturated condition there was no spray irrigation. Following saturation and stabilisation of the well pressure heads, drainage ports were opened instantaneously and the rate of inflow into the well was recorded. The infiltration rates, Qs, into the sand under the two soil conditions were multiplied by the appropriate A values from the FEA for each specific pressure head to derive Kfs. A values were significantly different under field-saturated and unsaturated conditions. Kfs of the sand was also measured independently in a constant-head permeameter. Values of Kfs from the FEA and the Glover approach were in close agreement with one another, and with that measured in the constant-head permeameter. Agreement was within 13% at the higher pressure heads. The procedure appears promising for field use.  相似文献   

14.
A combined methodology of basal crop coefficient (Kcb) derived from vegetation indices (VI) obtained from satellite images and a daily soil water balance in the root zone of the crop was proposed to accurately estimate the daily grape crop coefficient and actual evapotranspiration. The modeled values were compared with field measurements of crop evapotranspiration (ET) using an energy balance eddy-covariance flux tower and adjusted for closure using the measured Bowen ratio. A linear relation between Kcb and VI for vineyard was obtained, Kcb = 1.44 × NDVI-0.10 and Kcb = 1.79 × SAVI-0.08. The correlation of the measured crop coefficient (Kc) and modeled (Kcrf) exhibits a linear tendency, Kc = 0.96Kcrf, r2 = 0.67. Other derived parameters such as weekly Kc and daily and weekly ET show good consistency with measurements and higher coefficients of determination. The study of the soil water balance suggests the importance of soil water storage in grapes within the La Mancha region. These results validate the use of remote sensing as a tool for the estimation of evapotranspiration of irrigated wine grapes planted on trellis systems.  相似文献   

15.
The measurement or prediction of percolation losses in field situations is of great practical significance for efficient irrigation and for determination of the leaching requirement, particularly of clayey soils where impeded percolation occurs. Hydraulic properties and water losses in packed Ashutia clay soil were determined under prevented-evaporation and free-evaporation conditions using lysimeter and tensiometric techniques. Hydraulic conductivity was determined as a function of soil moisture content using percolation flux computed. An exponential relationship between hydraulic conductivity and soil water content K = aebθ, was found. The percolation and evaporation-plus-percolation fluxes estimated from tensiometer readings under prevented-and free-evaporation conditions, respectively, matched with profile water losses from lysimeter measurements. The error ranged between 0.01 and 0.82 mm day−1 with high correlation coefficient indicating that water loss from a soil profile can be estimated from tensiometer readings.  相似文献   

16.
The management of irrigated agricultural fields requires reliable information about soil hydraulic properties and their spatio-temporal variability. The spatial variability of saturated hydraulic conductivity, Ks and the alpha-parameter αvG-2007 of the van Genuchten equation was reviewed on an agricultural loamy soil after a 17-year period of repeated conventional agricultural practices for tillage and planting. The Beerkan infiltration method and its algorithm BEST were used to characterize the soil through the van Genuchten and Brooks and Corey equations. Forty field measurements were made at each node of a 6 m × 7.5 m grid. The soil hydraulic properties and their spatial structure were compared to those recorded in 1990 on the same field soil, through the exponential form of the soil hydraulic conductivity given by the Gardner equation, using the Guelph Pressure Infiltrometer technique. No significant differences in the results obtained in 1990 and 2007 were observed for either particle-size distribution or dry bulk density. The mean value of αvG-2007 was found to be identical to that of αG-1990, while that of Ks-2007 was significantly smaller than that of Ks-1990. In contrast to the Gardner equation, the van Genuchten/Brooks and Corey expression was found to be more representative of a well-graded particle-size distribution of a loamy soil. The geostatistical analysis showed the two parameters, Ks and αvG-2007, were autocorrelated up to about 30 and 21 m, respectively, as well as spatially positively correlated within a range of 30 m. Despite the difference in the mean values of Ks between the two studies, the spatial structures were similar to those found in the 1990 experiment except for the covariance sign. The similarity in autocorrelation ranges indicate that the spatial analysis of soil hydraulic properties is independent of the infiltration methods (i.e., measurement of an infiltration flux) used in the two studies, while the difference in the covariance sign may be linked to the use of two different techniques of soil hydraulic parameterization. The covariance values found in the 2007 campaign indicates a positive relationship between the two parameters, Ks and αvG-2007. The spatial correlations of soil hydraulic parameters appear to be temporally stabilized, at least within the agro-pedo-climatic context of the study. This may be attributed to the soil textural properties which remain constant in time and to the structural properties which are constantly renewed by the cyclic agricultural practices. However, further experiments are needed to strengthen this result.  相似文献   

17.
High value crops such as carrot planted in coarse soils of the Southern San Joaquin Valley in California are prime candidates for nitrate leaching through irrigation nonuniformity. A 2-year study was carried out to explore the impact of irrigation uniformity on nitrate leaching. Irrigation uniformity was measured using catchcans. Soil nitrate (NO3-N) and ammonium (NH4-N) contents were measured from soil sampled at different depths and times during two growing seasons. Nitrate leaching was determined using ion-exchange resin bags at 1-m depth sampled three times during each season. Although, soil NO3-N as well as seasonal irrigation was significantly higher along the lateral irrigation pipe than between the sprinklers, nitrate leaching was not significantly higher. As expected, soil nitrate content decreased as percolation increased for both years. Nitrate leaching, as estimated by anion-exchange resin bags, was positively correlated to soil NO3-N content but was not correlated to irrigation depth, irrigation uniformity, or deep percolation. Field variation in saturated hydraulic conductivity (Ks), soil organic matter (OM), and soil water retention at field capacity had limited effect on NO3-N and NH4-N distributions in the profile and on nitrate leaching. The results of this experiment suggest that irrigation nonuniformity has less impact on nitrate movement than suggested by earlier studies.  相似文献   

18.
Hydraulic conductivity (K) and soil water diffusivity (D) characterizing water flow under saturated and unsaturated conditions, respectively, were determined for a sandy loam and a clay loam soil, using water with different combinations of total electrolyte concentrations, C (i.e., 20, 40, 80, 125 and 250 meq 1?1) and sodium adsorption ratios, SAR (i.e., 0, 20, 30, 40, 80 and ∞ mmole l?12). Both K and D were found to increase with C and decrease with SAR. In low sodium adsorption ratio ranges (i.e., up to 20) the requirement of electrolyte concentration to maintain relative hydraulic conductivity = 0.5 was relatively more for sandy loam than for clay loam soil. However, the trend for electrolyte concentration requirements for the two soils was reversed at high sodium adsorption ratios (i.e. > 20). A spline function was used to draw the best fitting line through the data points of horizontal absorption experiments.  相似文献   

19.
Plant age and size, seasonal growth patters and crop load, among other factors, have been reported to decrease the usefulness of trunk diameter variation (TDV) derived indices as water stress indicators in olive trees. Our hypothesis, however, is that indices derived from TDV records in old, big olive trees are sensitive enough to detect levels of water stress in trees of orchards under deficit irrigation that, although severe, are below the threshold for fruit shrivelling. This is of importance for the production of good quality oils, since fruit shrivelling may affect oil quality. The aim of this work was to assess different TDV-derived indices as water stress indicators in 40-year-old ‘Manzanilla’ olive trees with heavy crop load. We derived the maximum daily shrinkage (MDS), daily growth (DG) and daily recovery (DR) from TDV records taken during the 2008 dry season both in well-irrigated FAO trees and in deficit-irrigated RI trees. Measurements of volumetric soil water content (θv), leaf water potential (Ψl), stomatal conductance (gs), net CO2 assimilation rate (A), water and oil accumulation in the fruits and yield parameters were made for both treatments. The trunks did not grow during the experimental season, either in the FAO or RI trees, likely because of the heavy crop load. Therefore, DG was useless as water stress indicator. For MDS and DR, which were responsive to the increase of the trees’ water stress, we calculated the variability, quantified by the coefficient of variation (CV), the signal intensity (SI) and the sensitivity (SI/CV) values. In addition, we derived reference equations for irrigation scheduling from the relationships between MDS values in the FAO trees and main meteorological variables. Values both of SI-MDS and SI-DR were steady until September 9, despite of increasing differences in θv between treatments from early in the dry season. The Ψl vs θv values showed an outstanding capacity of the RI trees to take up water from the drying soil, and the Ψl vs gs values showed a near-isohydric behaviour of those deficit-irrigated trees. These results explain, at least in part, the lack of response of MDS and DR on that period. Both SI-MDS and SI-DR peaked for the first time on September 9, 16 days before the appearance of fruit shrivelling. Our results suggest that using TDV-derived indices as water stress indicators for irrigation scheduling in old olive orchards with medium to low plant densities, i.e. with large root zones, may be useless in case the irrigation strategy is aimed at keeping the soil close to field capacity. Nevertheless, the MDS and DR indices may be useful indicators for the avoidance of fruit shrivelling in deficit irrigated olive orchards for the production of good quality oil. Reliable reference equations for scheduling irrigation with the signal intensity approach were obtained from the regression of MDS values vs the daily maximum values of both the air temperature and the vapour pressure deficit of the air.  相似文献   

20.
We present the results from a sensitivity analysis and a preliminary short-term, site-scale performance assessment of the analytical soil and groundwater nitrate transport RISK-N. The study was carried out in the Central Valley of Chile, on a 2.6 ha corn (Zea mays L.) field underlain by a shallow unconfined aquifer during the cropping season 2000–2001. Nitrogen levels in soils as well as NO3−–N irrigation water and groundwater concentrations were monitored through the crop-growing period, the latter by a network of 16 monitoring wells. A sensitivity analysis shows that both the nitrate flux from the vadose zone and NO3−–N groundwater concentration are mainly influenced by the initial soil nitrogen levels, water input, and soil porosity. Also, simulated groundwater NO3−–N levels are sensitive to changes on the saturated zone denitrification constant. An additional analysis further reveals the significance of the latter parameter, in conjunction with the amount of applied nitrogen fertilizer. We obtained a good agreement between observed average and simulated values. While the model performs well when spatially averaged values are used (root mean square error, RMSE = 1.4 mg l−1 of NO3−–N), the prediction error increases (RMSE = 1.9 mg l−1 of NO3−–N) when the concentration in each well is considered. This fact could be explained by the time and space scale of the experiment and the characteristics of the RISK-N model. The model is easy to use and seems appropriate for mid- and long-term studies of nitrogen contamination in groundwater for agricultural conditions in the Central Valley of Chile and under limited field data availability conditions. However, it needs to be tested for longer periods and under different climatic conditions, soil types, and aquifer characteristics, before its range of applicability can be fully established and recognized.  相似文献   

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