Measurement of water-use by Jatropha curcas L. using the heat-pulse velocity technique

In response to the proposed introduction of the potential bio-diesel species Jatropha curcas (Linnaeus) to South Africa, field experiments were conducted to investigate its likely water-use impacts relative to other forms of vegetative land use. As no existing water-use data could be found for this species worldwide, sap flow in Jatropha curcas trees was measured continuously for a 17-month period at two sites in eastern South Africa. These consisted of young (4-year-old) trees at a relatively wet site and mature (12-year-old) trees at a dry site. The heat-ratio method of the heat-pulse technique was utilised, together with measurements of meteorological variables and soil water. Sap- flow rates varied according to tree age, season, prevailing meteorological conditions, and soil moisture levels. Peak sapflow rates occurred during the warm wet summer months, but due to the deciduous nature of the species, water use was negligible during winter. Scaled-up sap-flow measurements resulted in estimates of total annual transpiration of 1 983 ℓ (147 mm) for a 4-year-old J. curcas tree, and 4 884 ℓ (362 mm) for a 12-year-old J. curcas tree. The study concluded that the J. curcas trees studied were conservative in their water use, and were unlikely to transpire more water than indigenous vegetation types of the area.Keywords: heat ratio method, sap flow, transpiration, water resource impact

Characterising the water use and hydraulic properties of riparian tree invasions: A case study of Populus canescens in South Africa

Invasive alien plants (IAPs) pose a serious threat to the already limited water resources in dry countries like South Africa which are facing increasing water shortages. Much of South Africa is expected to get drier in future due to climate change. In addition, the future climatic conditions are also predicted to accelerate the rate at which IAPs will spread, due to favourable growing conditions, further disrupting the provision of goods and services. Previous studies on tree water use in South Africa focused on commercial forests of introduced genera mainly Pinus, Eucalyptus, and Acacia. This study sought to expand these observations by quantifying water use and its drivers in riparian Grey Poplar (Populus canescens) invasions in the Berg River catchment of South Africa. Whole tree hydraulic resistance ranged from ~ 1.4 MPa·h·g-1 for large trees to ~14.3 MPa·h·g-1 for the small ones. These resistances are higher than those found for poplars in temperate climates, suggesting substantial hydraulic constraints to transpiration. Daily peak transpiration varied from 5 to 6 L·tree-1 in small trees (~9.2 cm DBH) to between 35 and 40 L·tree-1 for large trees (~24 cm DBH). Stand-level transpiration peaked at ~4 mm·d-1 in summer (Jan– Feb). However, the annual total transpiration was only 338 mm due to the deciduous nature of the species and also the high hydraulic resistance in the transpiration stream. Daily transpiration was strongly correlated to solar radiation (R2 > 0.81) while the air vapour pressure deficit (VPD) constrained transpiration at high VPD values. We conclude that the water use of the poplar invasions is significantly lower than that of other riparian invasions. The impact of these invasions on the water resources is therefore likely quite low, warranting less priority in alien plant clearing operations aimed at salvaging water

Are simple empirical crop coefficient approaches for determining pecan water use readily transferrable across a wide range of conditions?

The accurate estimation of evapotranspiration (ET) of orchard crops is critical for judicious irrigation water management and planning. However, it is impossible to measure ET under all possible combinations of climate and management practices, which necessitates the use of ET models. Although empirical models are more likely to be adopted by consultants and growers, due to easier parameterization and the requirements for fewer, more easily measured input parameters, they may not always be transferable across a wide range of conditions. As a result these models may not always give acceptably accurate ET values outside of the area in which they were calibrated. This study therefore aimed to evaluate empirical crop coefficient models for pecans in two different orchards which differ in climate and/or fractional canopy cover from where the models were developed. When testing the FAO-56 approach it was found that pecans should not be grouped under stone fruit and that a six stage crop growth should be considered, instead of the traditional four stage curve. Improved accuracy in estimating ET of pecans could, however, be achieved by using a pecan specific reference crop coefficients for a mature orchard and scaling this with fractional canopy cover for different orchards, provided that an adjustment was made for the influence of climate on canopy development. This was achieved by using a published growing degree (GDD) day crop coefficient relationship, provided seasonal accumulated thermal time is below 1600 GDD and that crop coefficients do not exceed 1.1.The Water Research Commission (Project K5/1770) with co-funding from the National Department of Agriculture, Forestry and Fisheries of South Africa.http://www.actahort.orgam2017Plant Production and Soil Scienc

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

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 (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.South Africa‟s Water Research Commission (Project K5/1770, Water use of fruit tree orchards), with cofounding from the South African National Department of Agriculture, Forestry and Fisheries.http://link.springer.com/journal/2712016-12-30hb201

Environmental horticulture for domestic and community gardens—An integrated and applied research approach

Societal Impact Statement Daunting global challenges of climate change and biodiversity loss may seem overwhelming. However, gardeners have a secret weapon—gardens, balconies, indoor planting, yards and allotments are mini-ecosystems that offer opportunities to counter perceptions of helplessness, inadequacy and resultant inaction by using those spaces to ‘Do what we can, with what we have, where we are’. Minimising gardening ‘footprints’ to mitigate harmful impacts, whilst maximising gardening ‘handprints’ to enhance benefits, is readily achievable. With this in mind, the Royal Horticultural Society is leading research into environmental horticulture for gardens, and benefits for individual wellbeing. Summary This article presents an integrated and applied research approach to the unique and multi-disciplinary area of science referred to here as environmental horticulture. It does this by: (a) providing an institutional perspective (The Royal Horticultural Society) on a research approach for this particular area, emphasising why domestic and community gardens are important in the context of global environmental threats; (b) presenting four primary research focus areas and project examples; and (c) highlighting interdisciplinary linkages, future research needs, public engagement/knowledge sharing opportunities, and ‘Green Skills’ development in the area of environmental horticulture. Research focus areas discussed are: (1) responding to the changing climate (adaptation, mitigation and resilience solutions in gardens); (2) ‘plants for purpose’ (harnessing the potential of horticultural plant diversity, and gardening, to help regulate environmental conditions); (3) sustainability and climate risk reduction through effective and efficient resource management (reduction, re-use, recycling and repurposing); and (4) gardening and cultivated plant choice for human health and wellbeing. We argue that a key research priority is improving our understanding of the linkages and interactions between soil, water, plants, weather and people. These crucial linkages affect above and below ground processes, for both outdoor and indoor plants. They impact the effectiveness with which water and nutrient cycling takes place, the extent to which ecosystem services may be delivered, and the resultant capacity of gardens and gardening to provide environmental and human health benefits

Modelling water use of subtropical fruit crops : the challenges

Subtropical fruit crops form an important part of the fruit industry in many countries. Many of these crops are grown in semi-arid regions or subtropical regions where rainfall is seasonal and as a result the vast majority of these perennial, evergreen orchards are under irrigation. This represents a significant irrigation requirement and with more emphasis being placed on the conservation of water and orchard profitability, it is becoming increasingly important to accurately estimate water use of these crops and schedule irrigation accordingly. The FAO-56 procedure is a simple, convenient and reproducible method for estimating water use. However, the transferability of crop coefficients between different orchards and growing regions is not always readily achieved, due largely to differences in canopy size and management practices. In addition, as subtropical crops tend to exhibit a higher degree of stomatal control over transpiration than most other agricultural crops, some measure of canopy or leaf resistance must be taken into account when using models based on atmospheric demand. The challenge is therefore to provide reliable and dynamic estimates of canopy resistance from relatively simple parameters which can be of use to irrigation consultants and farmers for determining the water requirements of these crops. The challenge remains to ensure that these dynamic estimates are realistic and readily applicable to a number of growing regions. The derivation of transpiration crop coefficients, based on canopy cover and height and a dynamic estimate of leaf resistance, provided reasonable estimates of transpiration in three orchards in contrasting climates, suggesting that this approach could prove useful in future for subtropical crops.http://www.actahort.org2018-05-10hj2017Plant Production and Soil Scienc

Use of linear regression models to estimate missing heat pulse velocity data: a case study in two citrus cultivars

Heat pulse velocity sap flow measurement techniques are considered reliable and accurate for estimating tree water use in woody species. In order to be representative and scalable, probes are typically installed in a number of sample trees and inserted to stratified depths to account for heterogeneity in hydraulic conductivity exhibited by the sapwood. However, during measurement of heat pulse velocities with a large number of probes for an extended period of time, periods of missing data in one or more probes are likely to occur. In this work the effectiveness of using linear regression models between different probe data sets to estimate and patch missing data was evaluated. Hourly heat pulse velocities in two well-irrigated citrus orchards planted with ‘Delta’ Valencia and ‘Bahianinha’ Navel orange trees [Citrus sinensis (L.) Osbeck] were measured using the heat ratio method (HRM). The measurements were conducted in two consecutive seasons at Moosrivier Farm in Groblersdal, which is located in the summer rainfall area of South Africa, as part of a Water Research Commission funded research project on water use of fruit trees. Four trees were sampled in each orchard, each of which had four probes inserted to different depths in the sapwood of the main stem. Two approaches were evaluated for the linear regression models. The first approach was to analyse the relationships between HPV data measured by the different probes. The second approach was to use equations developed between one probe and the average of the other probes in the same orchard at the same time of the day. The regression models were validated by comparing estimated values against independently observed data from the same probe. In both orange tree cultivars sap flow velocities were observed to decrease with increasing sapwood depth, measured from the bark towards the stem centre. This emphasizes the need to use multiple sensors to accurately estimate water use of individual trees when using sap flow measurement techniques. Linear regression models can be used to effectively estimate missing data when long term sap flow measurements using multiple probes are conducted in citrus trees

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

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

A new approach to modelling streamflow reductions resulting from commercial afforestation in South Africa

The Department of Water Affairs and Forestry has, for some time, needed a comprehensive tool to incorporate the impacts of commercial afforestation on water resources into water use authorisation and allocation processes. Simulation modelling on a national scale, using the ACRU model, was identified as one possible solution. However, in order to establish confidence in the national simulation results, it was necessary to first verify model output against reliable observed data from process studies and long-term catchment afforestation experiments. In the verification phase, long term reductions in streamflow resulting from afforestation were satisfactorily simulated for five research catchments. Some problems were experienced with catchments in the Western Cape, with verifications on shorter duration experiments and the simulation of specific evaporative processes. Modelling of low flows was less successful than for total flows. In the second phase (the generation of the national database), simulations were first performed for the dominant Acocks (1988) veld type (i.e. 0% afforestation) within 843 Quaternary Catchments exhibiting afforestation potential (to determine baseline streamflow), followed by simulations of streamflow after 100% afforestation with eucalyptus, pine and wattle respectively. The difference between the unafforested and afforested simulations equated to streamflow reduction caused by afforestation. This culminated in the generation of maps and tables expressing reductions in streamflow per Quaternary Catchment, which represents a working solution for immediate application but may be improved upon with further work.Articl

Modelling water use of subtropical fruit crops: the challenges

Subtropical fruit crops form an important part of the fruit industry in many countries. Many of these crops are grown in semi-arid regions or subtropical regions where rainfall is seasonal and as a result the vast majority of these perennial, evergreen orchards are under irrigation. This represents a significant irrigation requirement and with more emphasis being placed on the conservation of water and orchard profitability, it is becoming increasingly important to accurately estimate water use of these crops and schedule irrigation accordingly. The FAO-56 procedure is a simple, convenient and reproducible method for estimating water use. However, the transferability of crop coefficients between different orchards and growing regions is not always readily achieved, due largely to differences in canopy size and management practices. In addition, as subtropical crops tend to exhibit a higher degree of stomatal control over transpiration than most other agricultural crops, some measure of canopy or leaf resistance must be taken into account when using models based on atmospheric demand. The challenge is therefore to provide reliable and dynamic estimates of canopy resistance from relatively simple parameters which can be of use to irrigation consultants and farmers for determining the water requirements of these crops. The challenge remains to ensure that these dynamic estimates are realistic and readily applicable to a number of growing regions. The derivation of transpiration crop coefficients, based on canopy cover and height and a dynamic estimate of leaf resistance, provided reasonable estimates of transpiration in three orchards in contrasting climates, suggesting that this approach could prove useful in future for subtropical crops