ResearchSpace :: Browsing by Author "Ntshidi, Zanele"

Browsing by Author "Ntshidi, Zanele"

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Assessing water use by Prosopis invasions and Vachellia karroo trees: Implications for groundwater recovery following alien plant removal in an arid catchment in South Africa
(Elsevier, 2017-08) Dzikiti, Sebinasi; Ntshidi, Zanele; Le Maitre, David C; Bugan, Richard DH; Mazvimavi, D; Schachtschneider, K; Jovanovic, Nebojsa; Pienaar, Harrison H
Prosopis spp., (or Mesquite), is a desert adapted woody weed which has invaded large parts of arid and semi-arid regions. It has a characteristic deep tap root, and forms dense impenetrable thickets along river banks, flood plains, and seasonal water courses. Several studies have quantified the water use by Prosopis, but there is limited information on how this compares with that of co-occurring groundwater dependent indigenous tree species in the arid and semi-arid tropics. Consequently the effects of removing Prosopis on groundwater, where they co-occur with indigenous trees, are not known. This study quantified the incremental water use by Prosopis invasions compared with the indigenous deep rooted Vachellia karroo (V. karroo) trees that normally replace Prosopis once it has been cleared. The study was done in an arid catchment in the Northern Cape Province of South Africa. Stem sap flow dynamics, groundwater levels, and weather data were collected hourly over 15 months in the invaded catchment. Data collection continued for a further 11 months after Prosopis had been cleared to monitor the recovery of the ecosystem from the invasions. There was no significant difference (P = 0.116) in the sap flux density of the two species although Prosopis had a much thinner sapwood depth than V. karroo due to its ring porous wood anatomy. However, at the stand scale Prosopis transpired more than five times more water (~544 mm y(sup-1)) than V. karroo (~91 mm y(sup-1)) since there were six times more Prosopis than the indigenous species. The removal of Prosopis slowed down the rate of decline of the water table from a pre-clearing peak of ~8.9–5.0 mm d(sup-1) after alien plant removal. Tree transpiration was accurately predicted by a simple model that used aquifer properties and hourly changes in water table depths with a RMSE of ±0.22 mm d(sup-1) before and ±0.32 mm d(sup-1) after clearing Prosopis.
Characterising the water use and hydraulic properties of riparian tree invasions: A case study of Populus canescens in South Africa
(Water Research Commission, 2018-04) Ntshidi, Zanele; Gush, Mark B; Dzikiti, Sebinasi; Le Maitre, David C
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(sup)-1 for large trees to ~14.3 MPa·h·g(sup)-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(sup)-1 in small trees (~9.2 cm DBH) to between 35 and 40 L·tree(sup)-1 for large trees (~24 cm DBH). Stand-level transpiration peaked at ~4 mm·d(sup)-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.
A comparative assessment of water use by Acacia longifolia invasions occurring on hillslopes and riparian zones in the Cape Agulhas region of South Africa
(Elsevier, 2018-10) Mkunyana, YP; Mazvimavi, D; Dzikiti, Sebinasi; Ntshidi, Zanele
The detrimental impacts of invasive alien plants on ecosystems and water resources have raised concerns in arid and semi-arid countries like South Africa where the average precipitation is approximately 500 mm/yr, which is below the world average of around 860 mm/yr. Several studies have examined the effects of invasive alien plants such as the Australian Acacias on the water resources. However, few studies have quantified the differences in water use between hillslope and riparian Acacia longifolia invasions. A. longifolia is one of the aggressive invader species in South Africa even on hillslopes that contribute substantially to runoff generation. Therefore, the encroachment of invasive alien plants has the potential to reduce runoff, thereby adversely affecting the available water downstream. This paper aims to; 1) compare transpiration rates of A. longifolia growing on hillslopes and along riparian areas, 2) establish the key drivers for water use by this species, and 3) estimate the hydrological impacts of the invasions at the catchment scale in the Heuningnes catchment, in the Western Cape Province of South Africa. Transpiration by the trees was measured using the heat pulse velocity sap flow method. Automatic weather stations and soil moisture sensors were used to monitor weather and soil water content variations at each site. The results showed that, at the stand level the riparian A. longifolia transpired two times more water (596 mm/yr) than on the hillslope (242 mm/yr). During years with above average rainfall, the water use rates by the invasions was estimated to be 579 mm/yr on the hillslope and could be as much as 1348 mm/yr at the riparian site. Thus, the hypothesis that riparian trees use more water than invasions on non-riparian areas was accepted in this study. At the catchment scale (740 km2), the estimated water use by the invasions was 20.5 Mm3. Clearing of all the invasions in the study catchment is likely to make 17 Mm3/yr of water available. Hence the clearing of A. longifolia along the riparian corridors should be prioritised as this will lead to water savings at the catchment scale.
Comparison of two remote sensing models for estimating evapotranspiration: algorithm evaluation and application in seasonally arid ecosystems in South Africa
(Springer; Science Press, 2019-09) Dzikiti, Sebinasi; Jovanovic, Nebojsa; Bugan, Richard DH; Ramoelo, Abel; Majozi, Nobuhle P; Nickless, Alecia; Cho, Moses A; Le Maitre, David C; Ntshidi, Zanele; Pienaar, Harrison H
Remote sensing tools are becoming increasingly important for providing spatial information on water use by different ecosystems. Despite significant advances in remote sensing based evapotranspiration (ET) models in recent years, important information gaps still exist on the accuracy of the models particularly in arid and semi-arid environments. In this study, we evaluated the Penman-Monteith based MOD16 and the modified Priestley-Taylor (PT-JPL) models at the daily time step against three measured ET datasets. We used data from two summer and one winter rainfall sites in South Africa. One site was dominated by native broad leaf and the other by fine leafed deciduous savanna tree species and C4 grasses. The third site was in the winter rainfall Cape region and had shrubby fynbos vegetation. Actual ET was measured using open-path eddy covariance systems at the summer rainfall sites while a surface energy balance system utilizing the large aperture boundary layer scintillometer was used in the Cape. Model performance varied between sites and between years with the worst estimates (R2<0.50 and RMSE>0.80 mm/d) observed during years with prolonged mid-summer dry spells in the summer rainfall areas. Sensitivity tests on MOD16 showed that the leaf area index, surface conductance and radiation budget parameters had the largest effect on simulated ET. MOD16 ET predictions were improved by: (1) reformulating the emissivity expressions in the net radiation equation; (2) incorporating representative surface conductance values; and (3) including a soil moisture stress function in the transpiration sub-model. Implementing these changes increased the accuracy of MOD16 daily ET predictions at all sites. However, similar adjustments to the PT-JPL model yielded minimal improvements. We conclude that the MOD16 ET model has the potential to accurately predict water use in arid environments provided soil water stress and accurate biome-specific parameters are incorporated.
Contrasting water use patterns of two drought adapted native fruit tree species growing on nutrient poor sandy soils in northern KwaZulu-Natal
(2022-07) Dzikiti, Sebinasi; Ntuli, NR; Nkosi, NN; Ntshidi, Zanele; Ncapai, L; Gush, MB; Mostert, THC; Du Preez, R; Mobea, NT; Mpandeli, S; Pienaar, Harrison H
Natural forests are an important source of livelihood for various communities. Yet economically important fruit tree species endemic to sub Saharan Africa e.g. Sclerocaryea birrea (Marula) are critically endangered due to over-exploitation and climate change. Effective conservation, management and utilization of native fruit trees require accurate quantitative information on how the trees interact with their environment. In this study we investigated the water use patterns and fruit yield dynamics of S. birrea, and another widely used species Strychnos spinosa (Monkey orange) growing on nutrient poor sandy soils in northern KwaZulu-Natal Province, South Africa. The study aimed to understand how the water use and yield of these species are influenced by climate and soils. These data are essential to support biodiversity conservation and management, and agroforestry programs. Transpiration was measured on co-occurring trees of each species over two years using the heat ratio sap flow method. The sap flow and environmental data were used to develop and test a simple Penman-Monteith transpiration model. The model provided insights on how environmental factors affected water use and yield in the trees’ natural habitats. The two species had similar diurnal transpiration trends characterized by midday stomatal closure. Peak transpiration per unit leaf area was higher for S. spinosa (2.3 L/m2/d) than S. birrea (1.4 L/m2/d). S. birrea transpiration responded strongly to rainfall pulses with the soil water deficit explaining 65% of the variation in tree water use. S. spinosa transpiration, on the other hand, was largely driven by the atmospheric evaporative demand rather than by soil water availability (R2 < 0.10), suggesting that the two species had different mechanisms to cope with drought stress. Despite the contrasting water relations, transpiration could be determined accurately using the combination model. The model has potential applications in agroforestry and species-specific conservation programs by providing accurate information on their water requirements.
Deriving crop coefficients for evergreen and deciduous fruit orchards in South Africa using the fraction of vegetation cover and tree height data
(2023-08) Mashabatu, M; Ntshidi, Zanele; Dzikiti, S; Jovanovic, N; Dube, T; Taylor, NJ
Inaccurate crop coefficients are major contributing sources of uncertainty that lead to inefficient use of limited available water resources. Understanding the need to improve water use efficiency in South Africa’s fruit industry, this study evaluated the method of deriving crop coefficients developed by Allen and Pereira (2009) over a variety of irrigated fruit tree crops. Detailed data of transpiration, evapotranspiration and weather variables measured using the heat ratio method, eddy covariance method and automatic weather stations, were collected from a water research funding body established by the South African government. This study adjusted the stomatal sensitivity function (Fr) in the model by replacing the ratio of the leaf resistance (rl) to the standard leaf resistance of a reference crop (100 sm-1) with rl/a where a is a resistance parameter for the specific crop. The resistance parameter was solved accordingly for each fruit type. Respective unique a values were obtained as: macadamia nuts (200 sm-1), citrus (50 s m-1), peaches (20 s m-1) and pecans (20 s m-1). These unique values were used to simulate basal and single crop coefficients that produced satisfactory results when compared to the actual measured values. Overly, no unique standard a value exists for most tree crops although a value close to 20 sm-1 may give reasonable estimates for pome and stone fruit. Crop coefficients derived using locally measured data were standardised and tabulated in a format that facilitates their transferability between sites. However, there is still a need to acquire crop specific information to parameterize a and improve accuracies.
Effect of different irrigation systems on water use partitioning and plant water relations of apple trees growing on deep sandy soils in the Mediterranean climatic conditions, South Africa
(2023-07) Ntshidi, Zanele; Dzikiti, S; Mazvimavi, D; Mobe, Nompumelelo T
All commercial apple fruit (Malus Domestica (Borkh) exported from semi-arid regions are grown under irrigation with drip and micro sprinkler systems being the most widely used. Few studies have directly compared the physiological responses of fruit trees to these systems in detail leading to uncertainties around their performance. This study investigated variations in transpiration rates, tree water status, growth, water use partitioning, fruit yield and quality for trees growing on deep sandy soils under these two systems. Data were collected in a mature Royal Gala orchard in South Africa over three growing seasons. Tree transpiration was quantified using the heat ratio method of monitoring sap flow while the soil water balance approach was used to derive the evapotranspiration (ET) rates. Leaf level results showed that one day after irrigation on hot dry days, the stomatal conductance was, on average, almost double for trees under micro than those under drip irrigation. There was more stress under drip with the minimum midday leaf water potential dropping to under -1.80 MPa compared to only around -1.20 MPa under micro sprinklers. Consequently, the tree transpiration per unit leaf area was substantially higher under micro sprinkler (2.9 L/m2/d) compared to 2.3 L/m2/d under drip (P = 0.05). Canopy growth was slower under drip with peak leaf area index (LAI) around 2.1 compared to 2.7 under the micro sprinkler system. The micro sprinkler system had a more active ground cover than the drip. At peak canopy cover in summer, up to 28% of ET was derived from the orchard floor under micro compared to only 15% under drip. However, fruit size and fruit quality were lower under drip compared to micro sprinkler irrigated trees. The study highlights that while water savings are high under drip irrigated orchards on sandy soils, trees tend to experience considerable water stress culminating in smaller fruit of compromised quality.
Estimating crop coefficients for apple orchards with varying canopy cover using measured data from twelve orchards in the Western Cape Province, South Africa
(Elsevier, 2020-04) Mobe, Nompumelelo T; Dzikiti, Sebinasi; Zirebwa, SF; Midgley, SJF; Von Loeper, W; Mazvimavi, D; Ntshidi, Zanele; Jovanovic, Nebojsa
The FAO-56 crop coefficient (Kc) approach is widely used for making irrigation decisions. Allen and Pereira (2009) extended this approach by developing a method for estimating Kc using a density coefficient (Kd), which is estimated from the fraction of ground covered by vegetation and plant height. In this study we evaluated this method using detailed measurements of transpiration (T), evapotranspiration (ET), soil attributes, weather, and tree physiological variables in 12 apple (Malus domestica Borkh.) orchards in the Western Cape Province of South Africa. Mid-summer canopy cover of the orchards was less than 20% in young non-bearing and exceeded 60% in mature full-bearing orchards. Data were collected over three growing seasons (October 2014 to May 2017) in orchards planted to the Golden Delicious/Reinders®, Cripps Pink, Cripps Red, and Rosy Glow apple cultivars. The original Allen and Pereira (A&P) method significantly overestimated the basal crop coefficients (Kcb) by on average 47% in mature and 103% in young orchards, respectively. However, improved Kcb estimates were obtained by adjusting the ratio of the resistances (i.e. rl /100) in the A&P method, where rl is the mean leaf resistance and 100s/m is the typical resistance for annual crops. We defined a resistance parameter “a” for apple orchards which is equivalent to the bulk canopy resistance of a well-watered tree. Replacing rl /100 with rl /a, and using the measured mean rl and other biophysical measurements to solve the A&P equation for a gave a value ~ 37 s/m. The improved Kcb values were used to derive the orchard Kc taking into account the contribution of cover crops whose transpiration was measured using miniature stem heat balance sap flow gauges. The seasonal total transpiration (T) estimated as T = Kcb x ET o, where ETo is the reference ET closely matched the measured values with a RMSE (root of the mean square error) of ~±16 mm. Therefore, using the mean canopy resistance which is representative of apple trees in the A&P method has the potential to accurately predict both the crop coefficients and water use of apple orchards from planting until full bearing age.
Estimating the water requirements of high yielding and young apple orchards in the winter rainfall areas of South Africa using a dual source evapotranspiration model
(Elsevier, 2018-06) Dzikiti, Sebinasi; Volschenk, T; Midgley, SJ; Lötze, E; Taylor, NJ; Gush, Mark B; Ntshidi, Zanele; Zirebwa, SF; Schmeisser, M; Jarmain, C; Steyn, WJ; Pienaar, Harrison H
Exceptionally high yielding (> 100 t ha-(sup1)) apple orchards (Malus domestica Borkh.) are becoming common in South Africa and elsewhere in the world. However, no accurate quantitative information currently exists on the water requirements of these orchards. Information is also sparse on the water use of young apple orchards. This paucity of data may cause inaccurate irrigation scheduling and water allocation decisions, leading to inefficient use of often limited water resources. The aim of this study was therefore to investigate the dynamics of water use in eight apple orchards in South Africa planted to Golden Delicious and the red cultivars i.e. Cripps’ Pink, Cripps’ Red and Rosy Glow in order to understand how canopy cover and crop load influence orchard water use. Four of the orchards were young (3–4 years after planting) and non-bearing, while the other four were mature high yielding orchards. Transpiration was monitored using sap flow sensors while orchard evapotranspiration (ET) was measured during selected periods using eddy covariance systems. Scaling up of ET to seasonal water use was done using a modified Shuttleworth and Wallace model that incorporated variable canopy and soil surface resistances. This model provided reasonable estimates in both mature and young orchards. The average yield in the two mature ‘Cripps’ Pink’ was ~110 t ha-(sup1) compared to ~88 t ha-(sup1) in the ‘Golden Delicious’ orchards. However, average transpiration (Oct-Jun) was ~638mm for the ‘Cripps’ Pink’ and ~778mm in the ‘Golden Delicious’ orchards. The peak leaf area index was ~2.6 and ~ 3.3 for the mature ‘Cripps’ Pink and ‘Golden Delicious’ orchards. So, canopy cover rather than crop load was the main driver of orchard water use. Transpiration by the young orchards ranged from 130 to 270 mm. The predicted seasonal total ET varied from ~900 to 1100mm in the mature orchards and it was ~500mm in the young orchards. Orchard floor evaporation accounted for ~18 to 36% of ET in mature orchards depending on canopy cover and this increased to more than 60% in young orchards.
Water use dynamics of young and mature apple trees planted in South African orchards: a case study of the Golden Delicious and Cripps’ Pink cultivars
(Copernicus Publications, 2018-05) Ntshidi, Zanele; Dzikiti, Sebinasi; Mazvimavi, D
Apple orchards have previously been bearing 60–80 t ha-1 at most. However in recent years yield has increased to more than 100 t ha-1. There is need to understand the water requirements of the high yielding orchards, given that high crop loads are associated with high water use rates. The aim of this study was to quantify the water requirements of young and mature unstressed apple orchards. We also assess the impact of climate variables on transpiration rates.
Water use efficiency of indigenous fruit trees in South Africa
(2022-07) Ntshidi, Zanele; Mobe, Nompumelelo T; Dzikiti, S
South Africa is endowed with many indigenous fruit tree species (IFTs). Besides Marula (Sclerocaryea birrea subsp. Caffra), use of indigenous fruit products in the country is generally low compared with other countries on the African continent. Indigenous fruit can play an essential role in alleviating poverty through income generation and improving food security especially in poor rural households. The main objective of this study was to measure the water use of indigenous fruit trees with domestication & commercialisation potential, monitor the fruit yield and eventually calculate the Water Use Efficiency (WUE). Tree transpiration was measured using the Heat Pulse Velocity method of the sap flow technique while fruit yield was obtained by manually counting the number of fruits. The WUE of indigenous trees was low compared to that of exotic species found in literature. Use of indigenous fruit products should be invigorated as these fruits thrive with minimal water demand while having potential to alleviate poverty.
Water use of selected cover crop species commonly grown in South African fruit orchards and their response to drought stress
(2021-12) Ntshidi, Zanele; Dzikiti, Sebinasi; Mazvimavi, D; Mobe, Nompumelelo; Mkunyana, YP
Cover crops are widely planted in orchards for a variety of reasons. These include suppressing soil erosion, nutrient cycling, phytosanitary purposes, general orchard aesthetics etc. However, there is need to balance these benefits against use of scarce resources such as water and nutrients. Currently no information exists on how different cover crop species use water in orchards and how they cope with drought stress. The aim of this study was therefore to compare the transpiration dynamics of various cover crop types in order to identify species with conservative water use rates. Studied species included: 1) two exotic legumes i.e. Lupine (Lupinus albus L.), and Common vetch (Vicia sativa), 2) three exotic grasses i.e. Tall fescue (Festuca arundinacea), Rye grass (Lolium perenne), and Kikuyu grass (Pennisetum clandestium) and; 3) grasses that are indigenous to sub-Saharan Africa i.e. African Lovegrass (Eragrostis capensis) and Rhodes grass (Chloris gayana). The crops were planted in pots under controlled greenhouse conditions. Transpiration rates were quantified using miniature stem heat balance sap flow gauges and by manual weighing. Drought stress was imposed by withholding irrigation at selected intervals and the responses were quantified through changes in the water relations of the plants. The study showed that exotic legumes had the highest daily water use which peaked at about 2.4 L per square metre of leaf area per day, followed by exotic grasses at 1.5–2.0 L/m2/d. The indigenous grasses used the least water ranging from 0.8 to 1.2 L/m2/d. The indigenous grasses largely displayed an isohydric response to drought stress by maintaining their leaf water status with increasing soil water deficit. The exotic species, on the other hand, showed risk taking behaviour (anisohydry) wherein both the transpiration and leaf water status decreased sharply as drought stress increased. Consequently, some exotic species failed to recover when stress was relieved. From a water use perspective, this study demonstrates that indigenous grass species are more appropriate as cover crops in South African orchards because of their low transpiration rates and the ability to cope with extended periods of water deficit.

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