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Item Characterisation and potential optimisation of seepage wetlands for nitrate mitigation in New Zealand hill country : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science, Massey University, School of Agriculture and Environment, Palmerston North, New Zealand(Massey University, 2023) Sanwar, SuhaDiffuse nitrate (NO₃⁻) loss to pastoral waterways in hill country headwater catchments is a water quality concern in many countries with pasture-dependent economies, including New Zealand (NZ). Sheep and beef farming is the dominant land use in NZ hill country which are often located in headwater catchments. As these primary industries strive toward production growth to meet global demand for meat exports, this agricultural intensification will introduce more NO₃⁻ to its waterways. This contrasts with the recently enacted National Policy Statement for Freshwater Management 2020 (NPS-FM) which recognises the significance and calls for the protection of small wetlands in recognition of their ecosystem services including nutrient regulation, water quality improvement as well as associated social well-being. Nitrate mitigation in low-order streams in pastoral headwater catchments are important due to their proportionally large catchment coverage and major contribution to the national NO₃⁻ load to NZ rivers. Seepage wetlands in hill country landscapes can be a N-sink and, therefore, is a potentially cost-effective and natural NO₃⁻-mitigation tool for improved water quality from the pastoral headwater catchments. Seepage wetlands are features that occur along low-order streams in the low gradient of hill country landscapes. Their organic matter-rich sediment, saturated conditions and locations at the convergence of surface and subsurface NO₃⁻ rich flow pathways make seepage wetlands a unique landscape feature in terms of NO₃⁻ reduction via denitrification processes. However, denitrification is spatially and temporally variable as the process is influenced by the wetland sediment and hydrological properties. Several studies have demonstrated that seepage wetlands can be a potential NO₃⁻ sink and have quantified high sediment denitrification capacities in individual wetlands. However, variations in sediment and denitrification properties across a range of wetlands and a comprehensive study of seepage wetland hydrological characteristics that influence NO₃⁻ attenuation have not been undertaken, particularly in pastoral hill country landscapes in NZ. This thesis has examined the spatial variabilities of seepage wetland denitrification and the denitrification-influencing sediment properties across four hill country seepage wetlands within the Horizons Regional Council administrative boundary in NZ. The spatial gradients of sediment properties were examined vertically (at 15 cm depth intervals) and horizontally (within- and between- wetlands) in seepage wetland sites. Sediment physicochemical (water content (WC), pH, Eh) and chemical properties (dissolved organic carbon (DOC), NO₃⁻, NH⁴⁺, %total carbon or %TC, %total nitrogen or %TN, C:N, dissolved Fe²⁺ and dissolved Mn²⁺) and sediment denitrification enzyme activity (DEA), that represents sediment denitrification capacity, were quantified. The DEA values were highest at the surface depths across all wetland sites. Based on the wide range (560-5371 µg N₂O-N kg⁻¹ DS h⁻¹) and distinctive surface DEA values, the seepage wetland study sites were categorised into high-performing H-DEA (>3000 µg N₂O-N kg⁻¹ DS h⁻¹) and comparatively low-performing L-DEA (<1000 µg N₂O-N kg⁻¹ DS h⁻¹) sites. The H-DEA sites measured 7 to 10 times higher surface DEA values compared to the L-DEA sites. Spatial variability of denitrification in seepage wetlands was mainly driven by sediment WC, NO₃⁻, %TC, %TN, C:N, dissolved Fe²⁺ and dissolved Mn²⁺ (p≤0.05). The H-DEA site measured high WC (78%) which was above the threshold for denitrification and high sediment NO₃⁻ (15.9-18.5 mg NO₃⁻N/kg DS), in contrast to the L-DEA sites (WC 39.8-37.4%, 2.5-3.97 mg NO₃⁻N/kg DS). The heterogeneity of WC explained the heterogeneous distribution of DEA within the individual L-DEA sites. The sediment properties accounted for only 58-73% of the overall spatial variability in DEA, suggesting that additional wetland characteristics such as wetland hydrology, could have an important influence on denitrification in seepage wetlands. The seepage wetland hydrology and associated NO₃⁻ removal were characterised in detail at one of the L-DEA sites located on Tuapaka farm. During the hydrological characterisation, streamflow discharge and water quality were monitored at inflow and outflow for a 2-year period (June 2019-May 2021). Shallow groundwater quality was monitored at the 0.5, 1 and 1.5 m depths at the inflow, midflow and outflow positions in the wetland for a 1.5-year period (November 2019-May 2021). The seepage wetlands site demonstrated a stream inflow-dominated hydrology (83-87%) with small seepage contributions (8-14%) to the seepage wetland hydrology. Precipitation was found to be the major hydrological and associated NO₃⁻ removal (means attenuation) driver in the seepage wetland site. The seepage wetland was found an overall NO₃⁻ sink that on an average removed 23% of the annual NO₃⁻ inflow. Compared to the stream inflow (<0.03 mg NO₃⁻N/L), higher shallow groundwater NO₃⁻ concentrations (<0.11 mg NO₃⁻N/L) suggests that seepage is potentially an important NO₃⁻ source in these wetlands. High flow conditions, high winter precipitation and direct grazing during low flow periods are potentially major NO₃⁻ loss hot moments. In contrast, initial rapid infiltration at the onset of high precipitation events in early winter and spring and dissipated flow conditions highlighted opportunities for NO₃⁻ attenuation in the wetland and were identified as major NO₃⁻ removal hot moments. An overall dissipated flow condition driven by seasonally equivalent precipitation (22% of annual precipitation in winter) facilitated considerably higher annual NO₃⁻ removal of 40.8% (2.78 kg NO₃⁻N) in the wetland in year 2, in contrast to very low NO₃⁻ removal (0.3%, ~0.02 kg NO₃⁻N) under an erratic annual precipitation distribution (38% of annual precipitation in winter) in year 1. These findings suggest there is scope to improve NO₃⁻ removal by optimising flow conditions to slow flow in seepage wetlands to minimise NO₃⁻ loss during NO₃⁻ loss hot moments. In a follow-up laboratory-scale seepage wetland intact sediment column experiment, the effectiveness of diffuse flow, via subsurface outflow, was investigated for the optimisation of the wetland NO₃⁻ removal. During the experiment, the flow intervention altered the NO₃⁻ reduction-constraints observed in the preceding hydrological study and facilitated anaerobic conditions conducive to denitrification to capitalise on the sediment denitrification capacity, which was quantified during the preceding seepage wetland sediment characterisation study. The flow intervention involved vertical downwelling of NO₃⁻ rich (5 mg NO₃⁻N/L) pastoral surface runoff and subsequent horizontal discharge through a subsurface sediment column depth of 15 cm depth, collected from the Tuapaka seepage wetland site. The effectiveness of the subsurface drainage intervention for NO₃⁻ removal was assessed by monitoring the subsurface outflow water quality. The study showed that flow intervention achieved 50-96% NO₃⁻ removal from NO₃⁻ rich surface runoff. Based on the observations from the column study, two separate optimal operational HRTs of 2 and 13 hr are recommended to achieve large NO₃⁻ removal (50% from NO₃⁻ input of 5 mg NO₃⁻N/L) in a short period of time and large reduction in NO₃⁻ concentration at the outflow (<0.15 mg NO₃⁻N/L), respectively. The reasonably short period of HRT for such high NO₃⁻ removal efficiency (50-96%) supports the potential for the application of subsurface outflow intervention as a practical in-situ NO₃⁻ mitigation strategy, which warrants further research. This study also acknowledges the associated technical limitations of translating the laboratory-based findings to the field scale and recommends future studies to overcome these research limitations including high sediment compressions during intact sediment column samplings from the field, for example. The thesis not only demonstrates a flow intervention strategy to improve NO₃⁻ mitigation via flow regulation in seepage wetlands, but also guides future management by identifying the potential seepage wetland hot spots in the landscape (chapter 3) and the NO₃⁻ removal hot moments in the wetlands (chapter 4) and also by recommending necessary HRTs for flow intervention (chapter 5). In summary, this thesis has generated a robust dataset that improves our understanding of seepage wetland characteristics and their influences on NO₃⁻ removal at spatial and temporal scales. From an application perspective, this research provides new knowledge as to ‘where’, ‘when’ and ‘how’ seepage wetlands can be targeted to enhance their role in NO₃⁻ mitigations in hill country landscapes. This information has the potential to accelerate the integration of seepage wetlands into the toolbox of NO₃⁻ management strategies that could be used at a farm scale to improve water quality leaving NZ pastoral headwater catchments.Item Modelling the long-term impact of modernized irrigation systems on soil water and salt balances, and crop water productivity in semi-arid areas under current and potential climate change conditions : integration of agrohydrological model, geographical information system, remote sensing, and climate change model : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University, Palmerston North, New Zealand(Massey University, 2022) Khan, Muhammad HamedIrrigated agriculture plays a key role in ensuring food security and rural livelihoods across semi-arid and arid regions, like in the Indus basin of Pakistan. However, the Indus basin irrigation system of Pakistan is facing serious threats of low crop yields and increasing water scarcity, waterlogging, soil salinity, and overexploitation of groundwater. Considering the irrigation water-management issues, water managers and policymakers in Pakistan are looking into the modernization of the irrigation practices by introducing sprinkler and drip irrigation systems with the intent to save water and enhance crop water productivity. However, such intervention if adopted at a larger scale could seriously affect regional soil water and salt balances, solute leaching, and recharge to groundwaters in semi-arid and arid regions. Therefore, a robust assessment of the long-term potential impacts of modernised irrigation systems, particularly under the potential climate change scenarios, is essential for improving productivity and sustainable irrigated agriculture in semi-arid and arid regions. Field experiments are practically difficult to quantify the long-term impacts of modernised irrigation practices on soil water and salt balances and crop growths, especially under projected climate change conditions. This thesis developed a modelling framework using local field experiments, and geographical and remote sensing information, combined with a spatially distributed agrohydrological model and climate change projections to analyse the potential impacts of different irrigation application scenarios at the field and canal command scales. This methodology is applied to evaluate the potential impacts of current and proposed modernized irrigation systems on soil water and salt balances, soil salinity build-up, percolation to groundwaters, crop yield and crop water productivity of irrigated crops under long-term contemporary climate (1987-2017) and potential climate change (2070-2099) scenarios. The main irrigated crops of wheat, rice, and cotton were studied in the Hakra branch canal command as a case study. The Hakra branch canal (HBC) command, located in the Indus basin irrigation system of Pakistan, covers 0.21 million ha and is characterised by the typical problems of canal water scarcity, poor groundwater quality, waterlogging and soil salinity, and less-than-optimal crop production. The information collected from local field-scale experiments during the years 2016-2017, GIS, remote-sensing techniques and global climate models are integrated to parametrise, calibrate, and validate the agrohydrological Soil-Water-Atmosphere-Plant (SWAP) model application at both field- and canal command- scales. The SWAP model simulated soil water and salt balances, percolation to groundwaters, and water- and salt-limited crop yields and crop water productivity values of main irrigated crops of wheat, rice, and cotton from field- to canal command- scales in the study area. The modelling assessment of current irrigation practices revealed significant variation in canal water supplies and over-exploitation of groundwater, resulting in high spatial variability in soil water percolation and salt build-up in the soil at the spatial scale of the head, middle and tail reaches of the canal command. The canal water-inflow is about 19% and 42% higher at the head reaches than at the middle and tail reaches, respectively. The significant seepage from the canal network and the cultivation of high water-consuming crops such as rice are the potential cause of waterlogging at the head reaches. Whereas limited canal inflow and use of poor-quality groundwater (> 3 dS m⁻¹) appear to be potential causes of soil salinity at the tail reaches of the HBC command. The detrimental effects of limited canal inflow and the use of marginal to poor groundwater causes considerable spatial variation in simulated water and salt-limited crop yields. The simulated water and salt-limited crop water productivity values are not only different for the different crops of wheat, rice and cotton, but also for the same crop across the study area. The field- and canal-command scale modelling was applied to simulate and assess the potential impacts of the proposed modernized irrigation scenarios, such as • sprinkler irrigation is defined as a high-efficiency irrigation system with leaching fraction (HEIS_LF) and without leaching fraction (HEIS_noLF), and • precision surface irrigation system (PSIS) for cotton-wheat cultivation under contemporary climate (1987-2017) and potential climate change (2070-2099) scenarios RCP 2.6 (low emission) and RCP 8.5 (high emission or business-as-usual). The long-term simulation results suggest a saving of about 40% in irrigation water under the HEIS_noLF scenario. However, this irrigation water-saving under the HEIS_noLF scenario resulted in the risk of an increase in soil salinity due to reduction in soil percolation and its associated salt build-up in the soil profile. Under the HEIS_noLF scenario for cotton-wheat cultivation, the soil salinity is simulated to increase from 2.6 to 8.0 dS m⁻¹ at the field-scale, and from 2 to >12 dS m⁻¹ at the canal command scale, affecting crop yields due to salt stress. The high salt build-up is simulated to reduce crop yields by 38% for cotton, and 48% for wheat under the contemporary climate (1987-2017) at the canal command scale. The soil salinity is simulated to get even worse in poor-quality groundwater areas, resulting in wheat failure of < 1 ton/ha with HEIS_noLF under the RCP 8.5 scenario of potential climate change (2070-2099) conditions. The modelling analysis suggests a significant leaching fraction is required to maintain acceptable soil salt balance for successful crop production. This leaching fraction could be achieved by a pre-sowing irrigation of 60 mm depth at the start of the season, followed by an additional 10 mm depth with each irrigation interval using a high-efficiency irrigation application, simulated as HEIS_LF. The HEIS_LF scenario resulted in 50 to 65% higher average water- and salt-limited crop water productivity values (kg/m³ ET) of 0.5 for cotton, and 1.87 for wheat. This is compared to the HEIS_noLF scenario of 0.25 for cotton, and 0.65 for wheat under potential climate change (2070-2099) conditions. However, the PSIS irrigation scenario resulted in similarly favourable soil water and salt balances, water and salt-limited crop yields and crop water productivity values for the cotton - wheat cultivation. Under the PSIS irrigation scenario, the average water-and salt-limited crop water productivity values (kg/m³ ET) are simulated as 0.50 for cotton and 2.79 for wheat under the contemporary climate (1987-2017), and 0.50 for cotton and 1.92 for wheat in potential climate change (2070-2099) conditions. The modelling analysis simulated the average soil percolation rate as 10 to 20% higher, resulting in the leaching of 20 to 30% more salts from the soil profile under the PSIS scenario than the HEIS_LF under potential climate change conditions. The key findings of this modelling assessment suggest that modernisation of irrigation systems as higher-efficiency (HEIS) irrigation applications, with no appropriate leaching fraction, would compromise salt build-up in the soil profile. This would potentially reduce crop yields and crop water productivity in the long-term, especially under potential climate change (2070-2099) conditions. There appears very limited scope for real irrigation water savings using a high-efficiency irrigation system for long-term sustainable crop production in areas making conjunctive use of limited canal water supplies and marginal- to poor-quality groundwaters. Hence, proposed initiatives for implementing high-efficiency irrigation systems should be carefully evaluated in terms of their long-term potential impacts on regional soil water and salt balances, crop yields and crop water productivity values in areas such as the Indus basin irrigation system in Pakistan, particularly under potential climate-change conditions.Item Transforming freshwater governing : a case study of farmer and regional council change in Hawke's Bay, New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Agriculture and Environment at Massey University, Manawatu, New Zealand(Massey University, 2021) Drury, Charlotte Josephine MaryAchieving improved freshwater governing and management is a global challenge, from which New Zealand is not exempt. Agriculture has played, and continues to play, a central role in New Zealand’s economy, but is also an activity that impacts freshwater. In this research it is argued that a transition is occurring in New Zealand that necessitates transformational change by both farmers and the entities that govern farmers’ freshwater management. This thesis explores at the micro (individual) level the lived experiences of two groups of regime actors involved in NZ’s freshwater governing transition: farmers, and regional councils – the governing entity that has the legislative responsibility to manage the freshwater resources of a region. The governing of farmers’ freshwater management in the Tukituki Catchment of the Hawke’s Bay region is the single case studied qualitatively. The research question answered is what is shaping the governing of farmers’ freshwater management, and what is shaping the regional council’s governing of farmers? The relationship between the two groups was of interest also. Data were primarily obtained through semi-structured interviews with farmers and people associated with the Hawke’s Bay Regional Council conducted between August 2016 and October 2017. Findings of this research suggest that the regional council was not actively governing farmer participants. Farmers were changing their freshwater management practices, but in response to broader societal pressures. Changes made were moderated by farmer networks and localised good farming norms linked with farmer identity. Freshwater was not at the time recognised as a component of good farming norms, nor a farmer’s identity. Farmer practices instead illustrated the ongoing dominance of a productivist logic. The transition for the regional council from an entity that historically had a hands-off approach to governing farmers and engaged with farmers through a productivist logic, to an entity that had an environmental protection logic and actively governed farmers required organisational transformation. It also necessitated a fundamental renegotiation of the relationship between farmers and the council. The challenges experienced by individuals and the organisation as a whole in adapting to a new formal institution that required transformational change arose from sticking points, institutional logics, ways-of-knowing, people’s self-identities and relationships. The depth of change necessary, individually and collectively, of farmers, natural resource management (NRM) governing entities and arguably others, explains why improvements in freshwater have not yet been fully realised. As explained by a farmer participant in this research it’s a hellova big job to do this stuff (F2).Item New sensing methods for scheduling variable rate irrigation to improve water use efficiency and reduce the environmental footprint : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University, Palmerston North, New Zealand(Massey University, 2020) El-Naggar, AhmedIrrigation is the largest user of allocated freshwater, so conservation of water use should begin with improving the efficiency of crop irrigation. Improved irrigation management is necessary for humid areas such as New Zealand in order to produce greater yields, overcome excessive irrigation and eliminate nitrogen losses due to accelerated leaching and/or denitrification. The impact of two different climatic regimes (Hawkes Bay, Manawatū) and soils (free and imperfect drainage) on irrigated pea (Pisum sativum., cv. ‘Ashton’) and barley (Hordeum vulgare., cv. ‘Carfields CKS1’) production was investigated. These experiments were conducted to determine whether variable-rate irrigation (VRI) was warranted. The results showed that both weather conditions and within-field soil variability had a significant effect on the irrigated pea and barley crops (pea yield - 4.15 and 1.75 t/ha; barley yield - 4.0 and 10.3 t/ha for freely and imperfectly drained soils, respectively). Given these results, soil spatial variability was characterised at precision scales using proximal sensor survey systems: to inform precision irrigation practice. Apparent soil electrical conductivity (ECa) data were collected by a Dualem-421S electromagnetic (EM) survey, and the data were kriged into a map and modelled to predict ECa to depth. The ECa depth models were related to soil moisture (θv), and the intrinsic soil differences. The method was used to guide the placement of soil moisture sensors. After quantifying precision irrigation management zones using EM technology, dynamic irrigation scheduling for a VRI system was used to efficiently irrigate a pea crop (Pisum sativum., cv. ‘Massey’) and a French bean crop (Phaseolus vulgaris., cv. ‘Contender’) over one season at the Manawatū site. The effects of two VRI scheduling methods using (i) a soil water balance model and (ii) sensors, were compared. The sensor-based technique irrigated 23–45% less water because the model-based approach overestimated drainage for the slower draining soil. There were no significant crop growth and yield differences between the two approaches, and water use efficiency (WUE) was higher under the scheduling regime based on sensors. ii To further investigate the use of sensor-based scheduling, a new method was developed to assess crop height and biomass for pea, bean and barley crops at high field resolution (0.01 m) using ground-based LiDAR (Light Detection and Ranging) data. The LiDAR multi-temporal, crop height maps can usefully improve crop coefficient estimates in soil water balance models. The results were validated against manually measured plant parameters. A critical component of soil water balance models, and of major importance for irrigation scheduling, is the estimation of crop evapotranspiration (ETc) which traditionally relies on regional climate data and default crop factors based on the day of planting. Therefore, the potential of a simpler, site-specific method for estimation of ETc using in-field crop sensors was investigated. Crop indices (NDVI, and canopy surface temperature, Tc) together with site-specific climate data were used to estimate daily crop water use at the Manawatū and Hawkes Bay sites (2017-2019). These site-specific estimates of daily crop water use were then used to evaluate a calibrated FAO-56 Penman-Monteith algorithm to estimate ETc from barley, pea and bean crops. The modified ETc–model showed a high linear correlation between measured and modelled daily ETc for barley, pea, and bean crops. This indicates the potential value of in-field crop sensing for estimating site-specific values of ETc. A model-based, decision support software system (VRI–DSS) that automates irrigation scheduling to variable soils and multiple crops was then tested at both the Manawatū and Hawkes Bay farm sites. The results showed that the virtual climate forecast models used for this study provided an adequate prediction of evapotranspiration but over predicted rainfall. However, when local data was used with the VRI–DSS system to simulate results, the soil moisture deficit showed good agreement with weekly neutron probe readings. The use of model system-based irrigation scheduling allowed two-thirds of the irrigation water to be saved for the high available water content (AWC) soil. During the season 2018 – 2019, the VRI–DSS was again used to evaluate the level of available soil water (threshold) at which irrigation should be applied to increase WUE and crop water productivity (WP) for spring wheat (Triticum aestivum L., cv. ‘Sensas’) on the sandy loam and silt loam soil zones at the Manawatū site. Two irrigation thresholds (40% and 60% AWC), were investigated in each soil zone along with a rainfed control. Soil water uptake pattern was affected mainly by the soil type rather than irrigation. The soil iii water uptake decreased with soil depth for the sandy loam whereas water was taken up uniformly from all depths of the silt loam. The 60% AWC treatments had greater irrigation water use efficiency (IWUE) than the 40% AWC treatments, indicating that irrigation scheduling using a 60% AWC trigger could be recommended for this soil-crop scenario. Overall, in this study, we have developed new sensor-based methods that can support improved spatial irrigation water management. The findings from this study led to a more beneficial use of agricultural water.