J. G. Annandale

Software for missing data error analysis of Penman-Monteith reference evapotranspiration

Abstract. The most common approach for the estimation of crop water requirements is to pair a crop factor with the evaporation from a reference surface. In this study, a user-friendly computer tool was developed to facilitate the calculation of daily FAO (Food and Agricultural Organization of the United Nations, Rome, Italy) Penman-Monteith reference crop evaporation (ET0), and to estimate errors that can arise if solar radiation, wind and vapour pressure data are not available. The ET0 calculator imports comma, tab or space-delimited daily weather data files in any user-specified format. It displays graphically and processes statistically, ET0 values calculated from full and incomplete weather data sets. The program is written in Delphi with a Paradox database and includes a comprehensive, context-sensitive help file. Sensitivity analyses were carried out for three locations as examples. The error in predicting ET0 using estimated weather parameters was reduced by using 5-day averages of ET0 rather than daily values. Although some error is incurred by estimating weather parameters, this is somewhat compensated for by the absence of any error that may have been associated with the measurements.

Predicting crop water uptake under full and deficit irrigation: An example using pea (Pisum sativum L. cv. Puget)

Abstract The traditional reference evaporation with empirical crop factor approach to irrigation scheduling can now be improved upon (due to the accessibility of personal computers) by using a more dynamic description of the factors affecting crop water uptake. The soil–water balance (SWB) model, which quantifies water uptake as a water-supply- or evaporative-demand-limited process, was successfully adapted to estimate the water-use of pea (Pisum sativum L. cv. Puget) under both well-watered and water-stressed conditions. A growth analysis experiment in Pretoria, South Africa, provided the necessary crop input parameters to the model. Simulations of soil water deficit and canopy growth compared well with independent data sets in a water-stress field trial. The model, developed in a user-friendly format, can be used as a generic crop irrigation scheduling tool, for full or deficit irrigation conditions, provided that specific crop growth parameters are known.

Modelling the long-term effect of irrigation with gypsiferous water on soil and water resources

The use of gypsiferous mine water for irrigation of agricultural crops is a promising technology that could solve problems related to both shortage of irrigation water and disposal of effluent mine drainage. The long-term effect of irrigation with lime-treated acid mine drainage on soil properties and catchment salt load was investigated. The soil water‐salt balance-crop growth model (SWB) and the CLIMGEN weather data generator were used to simulate 30 years of irrigation with gypsiferous mine water for different irrigation management scenarios, followed by 20 years of dry land summer cropping, to determine if the problem of salt disposal was merely being postponed. Generated weather input data were for Bethal (South Africa), soil input data for a Plinthic Ferralsol (FAO-UNESCO) and crop input data for a rotation of pearl millet (Pennisetum glaucumcv. SA Standard) and oats (Avena sativa L. cv. Overberg). The soil appeared to act as an effective salt sink, with large quantities of calcium sulphate (340‐404 Mg ha 1 ) being precipitated in 30 years, and with negligible amounts of remobilization thereafter. Dissolved salts in the soil solution increased during winter when rainfall is negligible. The highest concentrations occurred deeper in the profile, which is desirable because rooting is less dense there and plant growth is thus less affected. Due to large amounts of gypsum precipitation near the surface, the ions in solution were mainly Mg 2C and SO4 2 . Simulated root weighed, saturation electrical conductivities indicated that summer cropping should present no problems, but for certain winter crops a leaching fraction should be applied to obtain maximum yields. Drainage water quality was variable, depending heavily on rainfall. Peak salt levels were around 9.6 g L 1 . Depending on irrigation strategy, between 418 and 636 Mg ha 1 of salts was leached over 30 years. Once irrigation ceased, very little leaching occurred. Annual irrigation depended on rainfall, but averaged at about 1100 mm per annum. About 350 mm of percolation occurred, resulting in net utilization of around 750 mm annually. About 40% of the salts added through irrigation could be immobilized in the soil profile. It is concluded that year round, high frequency irrigation, with a leaching fraction in winter, should be an effective and economical means of utilizing large quantities of gypsiferous water without causing irreparable damage to soil resources. ©1999 Elsevier Science B.V. All rights reserved.

Shoot allometry of Jatropha curcas

The South African government has banned planting of Jatropha curcas L. (Jatropha), potentially a multipurpose tree and biofuel source, owing to insufficient knowledge about the species. Use of allometry as a non-destructive method of monitoring growth and biomass attributes of Jatropha was investigated. The objectives were to examine: reliability of allometry between above-ground variables and basal diameter and crown depth of Jatropha; effects of below-ground interspecies competition and tree spacing on allometry; and validity of these relationships with independent data. The study site was Ukulinga Research Farm, South Africa. Destructive sampling was carried out in March 2008, and tree height and basal diameter were measured periodically during March 2005 to April 2007. Regression analysis and analyses of covariance were used to analyse the data. The height-diameter equation developed by destructive sampling was validated using independent data. Highly significant allometric regressions resulted from using basal diameter (r ≥ 0.89) and crown depth (r ≥ 0.94). Stem diameter had linear relationships with wood and foliage biomass percentages (r = 0.91). Height-diameter equations were equivalent across competition and tree spacing treatments. Predicted and measured tree heights were linearly related (r > 0.97). It could be concluded that above-ground allometry of Jatropha was very reliable and not significantly affected by either below-ground interspecies competition or tree spacing. The site-specific allometric equations are useful for accurate and non-destructive estimations of Jatropha growth under various growing and (non-pruning) tree management conditions. The equations presented here are, however, not universally applicable.

Exporting large volumes of municipal sewage sludge through turfgrass sod production.

The nutrient content of sludge produced by municipal water treatment works often far exceeds the requirements of nearby crops. Transporting sludge further afield is not always economically viable. This study reports on the potential to export large volumes of anaerobically digested municipal sewage sludge through turfgrass sod production. Hypotheses examined are that sludge loading rates far above recommendations based on crop nutrient removal (i) are possible without reducing turf growth and quality, (ii) do not cause an accumulation of N and P below the active root zone, (iii) can minimize soil loss through sod harvesting, and (iv) do not cause unacceptably high nitrate and salt leaching. An 8 Mg ha(-1) sludge control (the recommended limit) was compared with sludge rates of 0, 33, 67, and 100 Mg ha(-1) on a loamy, kaolinitic, mesic, Typic Eutrustox soil near Johannesburg, South Africa. Sludge application rates up to 67 Mg ha(-1) significantly improved turfgrass establishment rate and color. The ability of sods to remain intact during handling and transport improved as the sludge application rate increased to 33 Mg ha(-1) but deteriorated at higher rates. A sludge application rate of 100 Mg ha(-1) was needed to eliminate soil loss, but this rate was associated with unacceptably high N leaching losses. All our hypotheses were accepted for application rates not exceeding 33 Mg ha(-1) on the proviso that some soil loss was acceptable and that the leaching fraction was carefully managed during the first 2 mo after sludge application.

A two-dimensional water balance model for micro-irrigated hedgerow tree crops

Distribution of water and energy is non-uniform in widely spaced, micro-irrigated, hedgerow crops. For accurate water use predictions, this two-dimensional variation in the energy and water balance must be adequately accounted for. To this end, a user-friendly, two-dimensional, mechanistic soil water balance model (SWB-2D), has been developed. Energy is partitioned at the surface depending on solar orientation, row direction and canopy size, shape and leaf area density. Water is assumed to be distributed uniformly at the surface in the case of rainfall, whilst micro-irrigation only wets a limited portion of the field. Crop water uptake is calculated as a function of evaporative demand, soil water potential and root density. Evaporation is also calculated as being either limited by available energy or by water supply. Water is redistributed in the soil in two dimensions with a finite difference solution to the Richards’ equation. A field trial was set up to test the 2-D soil water balance model in a citrus orchard at Syferkuil (Pietersburg, South Africa). Model predictions generally compared well to actual soil water content measured with time domain reflectometry probes. Scenario modelling and analyses were carried out by varying some input parameters (row orientation, canopy width, wetted diameter and fraction of roots in the wetted volume of soil) and observing variations in the output of the soil water balance. The model holds potential for improving irrigation scheduling and efficiency through increased understanding and accuracy in estimating soil water reserves, since it accounts for the differing conditions in the under-tree irrigated strip and inter-row rainfed areas.

Crop coefficient approaches based on fixed estimates of leaf resistance are not appropriate for estimating water use of citrus

Abstract The estimation of crop water use is critical for accurate irrigation scheduling and water licenses. However, the direct measurement of crop water use is too expensive and time-consuming to be performed under all possible conditions, which necessitates the use of water use models. The FAO-56 procedure is a simple, convenient and reproducible method, but as canopy cover and height vary greatly among different orchards, crop coefficients may not be readily transferrable from one orchard to another. Allen and Pereira (Irrig Sci 28:17–34, 2009) therefore incorporated a procedure into the FAO-56 approach which estimates crop coefficients based on a physical description of the vegetation and an adjustment for relative crop stomatal control over transpiration. Transpiration crop coefficients derived using this procedure and fixed values for citrus did not provide good estimates of water use in three citrus orchards. However, when mean monthly leaf resistance was taken into account, good agreement was found with measured values. A relationship between monthly reference evapotranspiration and mean leaf resistance provided a means of estimating mean leaf resistance which estimated transpiration crop coefficients with a reasonable degree of accuracy. The use of a dynamic estimate of mean leaf resistance therefore provided reasonable estimates of transpiration in citrus.

Mobility and uptake of zinc, cadmium, nickel, and lead in sludge-amended soils planted to dryland maize and irrigated maize-oat rotation.

Sludge application to agricultural lands is often limited mainly because of concerns about metal accumulation in soils and uptake by crops. The objective of the study was to test the following hypotheses: (i) in the short to medium term (5-10 yr), the application of good-quality sludge according to crop N requirements will not lead to significant accumulation of water-soluble metal fractions in soil, (ii) mobility and uptake of metals is higher under irrigated than dryland systems, and (iii) metal concentrations in plant tissue could reach phytotoxic levels before the soil reaches environmental threshold levels. Field plots were arranged in a randomized complete block design comprising four replications of three treatments (0, 8, and 16 Mg ha yr anaerobically digested municipal sludge) planted to dryland maize and irrigated maize-oat rotation. Soil and plant samples were collected after 7 yr of treatment application for selected metal analyses. A large fraction of the Zn, Ni, and Pb in the soil profile was ethylenediaminetetraacetic acid extractable (46-79%). Saturated paste-extractable fractions of Cd and Pb were <1 mg kg. Plant uptake of Cd, Pb, and Ni under irrigation was double that for dryland systems. Concentrations of the metals considered in plant tissue of both cropping systems remained well below phytotoxic levels, except for Zn under dryland maize, which received 16 Mg sludge ha yr. Metal concentrations in the soil remained far below total maximum threshold levels. Therefore, hypotheses 1 and 3 were accepted for the metals considered, and hypothesis 2 was rejected for Zn.

Nitrogen mineralization from sludge in an alkaline, saline coal gasification ash environment

Rehabilitating coal gasification ash dumps by amendment with waste-activated sludge has been shown to improve the physical and chemical properties of ash and to facilitate the establishment of vegetation. However, mineralization of organic N from sludge in such an alkaline and saline medium and the effect that ash weathering has on the process are poorly understood and need to be ascertained to make decisions regarding the suitability of this rehabilitation option. This study investigated the rate and pattern of N mineralization from sludge in a coal gasification ash medium to determine the prevalent inorganic N form in the system and assess the effect of ash weathering on N mineralization. An incubation experiment was performed in which fresh ash, weathered ash, and soil were amended with the equivalent of 90 Mg ha sludge, and N mineralization was evaluated over 63 d. More N (24%) was mineralized in fresh ash than in weathered ash and soil, both of which mineralized 15% of the initial organic N in sludge. More nitrification occurred in soil, and most of the N mineralized in ash was in the form of ammonium, indicating an inhibition of nitrifying organisms in the ash medium and suggesting that, at least initially, plants used for rehabilitation of coal gasification ash dumps will take up N mostly as ammonium.

Dry matter yield and water use efficiency of five perennial subtropical grasses at four levels of water availability

The effect of four levels of water availability on the yield and water use efficiency (WUE) of five subtropical perennial grasses was evaluated in a small plot trial under a rain shelter. The soil profiles were brought to 100% (W4 = the control), 75% (W3), 50% (W2) and 25% (W1) of field capacity on a weekly basis. Under W4 (control) and W3 conditions, yields did not differ significantly from each other. Where water became less available (W2 and W1), yields were significantly reduced, in comparison with the control (W4). Buffel grass, a drought-tolerant species, produced yields (11.7–20.0t ha−1) under moderate to severe water restriction (W1, W2 and W3), which were comparable with yields obtained with adequate water (W4) for traditionally-irrigated grasses such as couch grass (12.0–15.8t ha−1) and kikuyu (5.6–11.8t ha−1). All five grass species tended to use water more efficiently under moderate to severe water-limiting conditions (W1 and W2) than under control conditions (W4). These results also indicate that traditionally drought-tolerant grass species should not be overlooked when identifying species for use under irrigation.