<p>In the midwestern United States, nitrogen (N) pollution of surface and groundwaters is a substantial threat to water quality because of its ecological and human health effects. Hypoxia in the Gulf of Mexico is primarily caused by N runoff within the Mississippi River basin, and nitrate in drinking water may negatively impact human health in both adults and children. </p><p>Agricultural tile drainage is a common practice that facilitates the transport of N from fields to streams. While the impacts of tile drainage have been studied extensively at the field scale, the impacts on hydrology, nutrient transport, and groundwater recharge are still uncertain at the watershed and landscape scales. </p><p>The overall goal of this thesis work is to assess how tile drainage affects landscape-scale connectivity, hydrologic travel times, and N transport across a large catchment in west-central Indiana using 10 years of bi-annual water chemistry and stable isotope data from a community science education event. Land use data and a previously developed travel time distribution (TTD) model were also incorporated to accomplish this goal. A secondary goal is to estimate seasonal differences in groundwater recharge in west-central Indiana using stable water isotope data from precipitation and groundwater samples. </p><p>Qualitative travel times derived from δ²H and δ¹⁸O variability support the idea that short travel times have greater nitrate concentrations than long travel times. Greater N concentrations are also observed during wetter conditions with increased connectivity. The results of the GIS TTD model support the hypothesis that increasing drainage intensity reduces travel times. Groundwater recharge appears negligible in Tippecanoe County using a traditional water balance approach, but an isotope mass balance approach suggests that about 55-65% of annual recharge occurs during the summer and may be linked to intense precipitation events. </p><p>This knowledge improves our understanding of N transport and hydrologic connectivity in tile drained landscapes. The results of this thesis also demonstrate the importance of drainage density for travel times and provide additional insight into the seasonality of groundwater recharge in west-central Indiana. </p>

A feasibility study of an irrigation development proposal should include an analysis of the effects of water supply conditions on the degree to which development objectives are expected to be realised. A method of making this analysis was developed based on procedures for solving two problems. These were; (a) optimally allocating a property's available supply of water among competing crops, and, (b) optimally controlling an open channel distribution system to meet temporally and spatially varying water demand. The procedure developed for solving (a) was applied. A stochastic dynamic programming procedure was developed to optimally schedule the irrigation of a single crop, subject to constraints on the timing of water availability and total application depth. A second procedure was developed, employing a constrained differential dynamic programming algorithm, for determining optimal irrigation schedules for use with variable application depth systems, and when several crops compete for an intra-seasonally limited supply of water. This procedure was called, as frequently as water supply conditions allowed, to provide short-term irrigation schedules in a computer simulation of the optimal irrigation of several crops. An application system model was included in these procedures to transform a crop water-use production function into the required irrigation water-use production function. This transformation was a function of the application device type and the mean application depth. From an analysis of the on-property effects of water supply conditions, it was concluded that in order to achieve high economic and irrigation efficiencies, water supply conditions must be sufficiently flexible to allow the application system operator to vary the mean application depth but not necessarily the time periods of water availability. Additionally, irrigation scheduling procedures which seek economically optimum strategies offer the potential to achieve a maximum level of net benefit at levels of water availability significantly lower than has previously been used for design purposes.

Sur la région de la Crau, le système l’irrigation gravitaire appliqué aux prairies de foin joue un rôle important dans le maintien du cycle hydrologique en étant le principal contributeur à la recharge de la nappe souterraine de la Crau (70 à 80% de la recharge). Dans le futur, des pressions sur la ressource en eau alimentée ce système d’irrigation risquent de s’accroître du fait des changements climatiques et de l’augmentation des autres usages de l’eau (domestiques,industriels, ..) et induisent la nécessité de l’optimisation de ce système afin de maintenir l’état des ressources en eau souterraines. Cette optimisation nécessite la connaissance des vrais quantités d’eau apportées à la parcelle qui sont mal connues, et de spatialiser ces quantités à l’échelle du territoire. Ainsi, l’objectif de ce travail est de développer une méthodologie permettant d’améliorer la quantification de ces volumes à deux échelle (parcellaire et régionale). Un système numérique incluant un modèle d’irrigation gravitaire dénommé ‘Calhy’ a été développé au cours de ce travail. Ce système permet de caractériser le fonctionnement des principaux processus intervenant dans ce système d’irrigation (infiltration de l’eau dans le sol et la propagation à la surface de la parcelle). Mais l’estimation de ces processus est limité par la connaissance de certains paramètres non mesurables, tels que la conductivité hydraulique à saturation du sol et la rugosité hydraulique de la surface parcellaire. Une analyse de sensibilité AS a été menée dans un premier temps au cours de ce travail afin de définir la contribution de la variation de chaque paramètre non mesurable sur la variance des variables de sortie. Les résultats montrent la possibilité d’estimation ces paramètres à partir des accessibles variables auxquels ils sont sensibles. Ainsi, une méthode d’inversion s’est basé sur les résultats d’AS, combine le modèle Calhy et un dispositif expérimental a été appliqué dans un second temps pour l’estimation paramètres et valider l’approche proposée. Les résultats montrent que cette approche est robuste et efficace pour estimer ces paramètres. A la fois les paramètres ont été issus pour démarrer le système (estimés ou mesurés), nous avons étudié différentes modifications du système d’irrigation actuel (changement de la pente de la parcelle, du sens d’irrigation, l’apport de l’eau en différents points de la parcelle, ….), et leurs impacts sur l’homogénéité de l’infiltration et la durée de l’irrigation.296En parallèle, Nous avons établi un modèle empirique de dose d’irrigation basé sur l’analyse des pratiques d’irrigation investigués auprès des enquêtes procédés chez les agriculteurs. Différents modèles empiriques ont été développés en basant sur des régressions calculant la dose d’irrigation et la durée en fonction du débit disponible et des paramètres parcellaires caractérisant les conditions d’irrigation tels que la géométrie de la parcelle (longueur, largeur et surface). Le modèle de dose empirique investigué au cours de ce travail permet de fournir une estimation de la dose distribué sur tout le territoire de la Crau en intégrant ce modèle dans le simulateur de laCrau. / Worldwide, the irrigation accounts for 70% of all water consumption: understanding therelationship between irrigation and ecosystems and optimizing the irrigation practices cancontribute to the sustainability management of water resources. In the Crau region (southern ofFrance), the flooding irrigation system used for irrigating the hay fields plays an important role inwater cycle: in this system, considerable amounts of water are brought to the hay fields (about 20000 m3/ha/year i.e. 2 000 mm), which participate strongly to the recharge of the Crau aquifer(between 66% and 80% of the recharge). In the future, the pressures on the availability of waterresources that feed this irrigation system (the reservoir of Serre Ponçon) may increase because ofthe climate change and the increase in the another water uses. Thus, it becomes necessary tooptimize the irrigation practices in order to conserve the water and ensure a sufficient rechargefor aquifer of the Crau. This optimization requires i) the knowledge of the amount of waterbrought to the plot that are not currently known, ii) spatialize these amounts over the regionalscale. This work aims to develop a methodology to improve the quantification of these volumesat the field and regional scales. A numerical system that includes a flooding irrigation modelcalled 'Calhy' was developed, takes into account the main processes involved in this irrigationsystem (water infiltration into the soil and the runoff of water slide over the plot surface). Firstly,a sensitivity analysis was conducted in order to classify the Calhy’s parameters according to theirimportance and define an optimal experimental apparatus allowing to estimate them using aninverse approach. Secondly, an inversion procedure based on the proposed experimental297apparatus and the previous model was implemented on several plots in the study area. The resultsshow that the important parameters can be estimated and then Calhy can be used to analyse andoptimize irrigation practices. Then, different optimization scenarios were identified. In parallel,we developed an empirical model of irrigation dose based on the analysis of irrigation practicesin a group of exploitations in the study region. Different empirical models were developed;regressions were used to compute the irrigation dose and duration from geometricalcharacteristics of the borders (length, width and surface) and available water inflow rate. Theempirical model of irrigation dose developed in this work would provide a spatial estimation ofirrigation doses overall plots in the study region, and would contribute to a better quantificationof water recharge of the Crau aquifer and its locations.

<p>Eutrophication due to phosphorus (P) enrichment continues to be a primary water quality concern affecting freshwater and marine estuaries around the world. Excessive anthropogenic P inputs, driven by the need to meet the rising food and energy demands of a growing and increasingly urbanized population, have resulted in the buildup of P creating legacy (historical) P pools in agricultural landscapes. There is growing evidence that remobilization of accumulated legacy P can interfere with conservation efforts aimed at curbing eutrophication and improving water quality. Less is known about the magnitude and effects of these legacy P pools on dissolved reactive P (DRP) losses in tile-drained systems. This dissertation consists of three separate inquiries into how legacy P may affect DRP losses in tile drains. In the first inquiry, we examined the possibility of developing a suitable pedo-transfer function (pedoTF) for estimating P sorption capacity (PSC). Subsequent PSC-based indices (Phosphorus Saturation Ratio (PSR) and Soil Phosphorus Storage Capacity (SPSC)) were evaluated using daily water quality data from an in-field laboratory. The pedoTF derived from soil aluminum and organic matter accurately predicted PSC (R² = 0.60). Segmented-line models fit between PSR and soluble P (SP) concentrations in both desorption assays (R² = 0.69) and drainflows (R² = 0.66) revealed apparent PSR thresholds in close agreement at 0.21 and 0.24, respectively. Linear relationships were observed between negative SPSC values and increasing SP concentrations (R² = 0.52 and R² =0.53 respectively), and positive SPSC values were associated with very low SP concentrations in both desorption assays and drainflows. Zero SPSC was suggested as a possible environmental threshold. Thus, PSC-based indices determined using a pedoTF could estimate the potential for SP loss in tile drains. Also, both index thresholds coincided with the critical soil test P level for agronomic P sufficiency (22 mg kg^-1 Mehlich 3 P) suggesting that the agronomic threshold could serve as an environmental P threshold. In the second inquiry, PSC- based indices in addition to other site characteristics present in a P index (PI), were used as inputs in the development of a multi-layer feed-forward artificial neural network (MLF-ANN). The MLF-ANN was trained, tested, and validated to evaluate its performance in predicting SP loss in tile drains. Garson’s algorithm was used to determine the weight of each site characteristic. To assess the performance of ANN-generated weights, empirical data from an in-field laboratory was used to evaluate the performance of an unweighted PI (PI_NO), a PI weighted using Lemunyon and Gilbert weights (PI_LG), and an ANN-weighted PI (PI_ANN) in estimating SP losses in tile effluent. The MLF-ANN provided reliable predictions of SP concentrations in tile effluent (R² = 0.99; RMSE = 0.0024). Soil test P, inorganic fertilizer application rate (FPR), SPSC, PSR, and organic P fertilizer application rate (OPR), with weights of 0.279, 0.233, 0.231, 0.097, and 0.084, respectively, were identified as the top five site characteristics with the highest weights explaining SP loss in tile discharge. These results highlighted the great contribution of both contemporary and legacy P sources to SP concentrations in tile discharge. Also, PI_ANNwas the only PI with a significant exponential relationship with measured annual SP concentrations (R²= 0.60; p < 0.001). These findings demonstrated that MLF-ANNs coupled with Garson’s algorithm, can accurately quantify weights for individual site characteristics and develop PIs with a strong correlation with measured SP in tile discharge. Finally, in the third inquiry, we compared DRP loads and flow-weighted mean DRP (FDRP) concentrations in P source and P sink soils and evaluated the predominant DRP concentration – discharge (C-Q) behavior in these soils on a daily and event scale. At the daily scale, C-Q patterns were linked to the soil P status whereby a chemostatic and dilution behavior was observed for P source and P sink soils, respectively. At the event scale, C-Q patterns were linked to soil P status, flow path connectivity, and mixing of event water, matrix water, and rising shallow groundwater. The predominant anti-clockwise rotational pattern observed on P source soils suggested that, as the discharge event progressed, contributions from P poor waters including matrix and shallow groundwater resulted in lower DRP concentrations on the rising limb compared to the falling limb. However, the variable flushing and dilution behavior observed on the rising limb suggested that, in addition to discharge and soil P status, rapid exchanges between P pools, the magnitude of discharge events (Q), and the relative number of days to discharge peak (D_rel) also regulated DRP delivery. On the other hand, the predominant non-hysteretic C-Q behavior in P sink soils suggest that DRP loss from these soils can be discounted. Our collective results highlight the need for nutrient and conservation practices focused on P drawdown, P sequestration, and P supply close to the crop needs, which will likely be required to convert P sources to sinks and to avoid the conversion of P sinks to sources. </p>

<p>Drainage water recycling (DWR) is the practice of capturing, storing, and reusing subsurface drained agricultural water to support supplemental irrigation and has recently been proposed as a practice for improving the crop production and water quality performance in the tile-drained landscape of the U.S. Midwest. This study describes the development of a modeling framework to quantify the potential irrigation and water quality benefits of DWR systems in tile-drained landscapes and the application of the model using ten years of measured weather, tile drain flow and nutrient concentrations, water table, and soil data from two sites in the U.S. Midwest. From this modeling framework, the development and testing of an open-source online tool is also presented.</p><p></p><p>A spreadsheet model was developed to track water flows between a reservoir and drained and irrigated field area at each site. The amount of tile drain flow and associated nutrient loads that could be captured from the field and stored in the reservoir was estimated to calculate the potential water quality benefits of the system. Irrigation benefits were quantified based on the amount of applied irrigation annually. A reservoir size representing 6% to 8% of the field area with an average depth of 3.05 m was sufficient in meeting the annual irrigation requirements during the 10-year period at each site. At this reservoir size, average annual nitrate-N loads were reduced by 20% to 40% and soluble reactive phosphorus loads by 17% to 41%. Variability in precipitation within and across years, and differences in soil water characteristics, resulted in a wide range of potential benefits at the two sites.</p><p>An online tool was developed from the model, and a variance-based global sensitivity analysis was conducted to determine influential and low-sensitivity input parameters. The input parameter, depth of root zone, was the most influential input parameter suggesting that the estimation of total available water for the field water balance is a critical component of the model. Input settings describing the irrigation management and crop coefficients for the initial establishment and mid-season crop growth periods were also influential in impacting the field water balance. Reservoir seepage rate was influential in regard to the reservoir water balance, particularly at larger reservoir sizes. Sensitivity analysis results were used to develop a user-interface for the tool, Evaluating Drainage Water Recycling Decisions (EDWRD).</p><p>This study shows that DWR is capable of providing both irrigation and water quality benefits in the tile-drained landscape of the U.S. Midwest. The developed modeling framework supports future research on the development of strategies to implement and manage DWR systems, and the online tool serves as a resource for users to increase their awareness and understanding of the potential benefits of this novel practice.</p><p></p>

The Participatory Irrigation Management approach was introduced into Cambodia in 2000, which was called the Participatory Irrigation Management and Development (PIMD). The goal of PIMD is to establish Farmer Water User Communities (FWUCs) to take over the management of irrigation schemes in their district in order to improve the performance of irrigation schemes and farmers’ livelihoods. The implementation of FWUCs has resulted in both failure and success. Several studies have identified factors that influence the failure of FWUCs, but little research has focused on their success. By employing a single embedded case study approach, this research selected the most successful scheme in Cambodia to identify factors that influenced the success of the FWUC in irrigation management. The findings of this research could provide concrete assistance to the government, donors, and non-governmental organisations in improving the performance of less successful FWUCs in Cambodia. The result of this research showed that the success of the O-treing FWUC was influenced by five internal and two external factors. The internal factors were: 1) the level of local participation, 2) the governance and management of the scheme, 3) the value of the benefits that flow from the irrigation scheme, 4) the quality of the irrigation infrastructure, and 5) the characteristics of the farmer members within the scheme. The external factors were: 1) the level of external support provided to the scheme, and 2) market access. The success of the FWUC required farmer participation and this participation was enhanced when farmers obtained benefits from it. This research also found that access to markets was critical to make the benefits that flowed from the irrigation scheme more profitable to farmers, leading to farmer participation. Similarly, it was also important to make sure that the irrigation infrastructure was of a high quality to ensure the delivery of an adequate and timely supply of water to farmers so that they could grow crops that provided them with the benefits. This required external support from the Ministry of Water Resources and Meteorology, NGOs, and local authorities to help rehabilitate the scheme. External support was also critical for enhancing the governance and management of the scheme through assistance with the formation process, provision of financial resources, capacity building, rule enforcement, and conflict resolution. The governance and management of the scheme, in particular the leadership capacity of the FWUC was another critical factor because it ensured the maintenance and development of the irrigation infrastructure, the timely and adequate supply of water to farmers, farmers’ trust and respect for leaders, and farmer participation. Finally, the success of the FWUC could not be viewed independently from farmer characteristics within the scheme. Farmers tended to participate in irrigation management when they had a history of self-organisation, when they were relatively homogenous, and when they were dependent upon farming for their livelihoods. This research suggests that the successful implementation of FWUCs requires a focus on the seven factors and the interactions that occur between these factors. Irrigation stakeholders such as the Ministry of Water Resources and Meteorology, donors, NGOs, local authorities, local leaders, and farmers should work together to enhance these factors in order to ensure the success of FWUCs.

The last decade has been a period of great expansion and land use intensification for the New Zealand dairy farming industry with a 44% increase in national dairy cow numbers. Intensive dairy farming is now considered to be a major contributor to the deterioration in the quality of surface and ground water resources in some regions of New Zealand. Previous research has demonstrated intensive dairy farming is responsible for accelerated contamination of wateways by nutrients, suspended solids, pathogenic organisms and faecal material. A number of common dairy farming practices increase the risk of nutrient leaching. In particular, farm dairy effluent (FDE) has been implicated as a major contributor to the degradation of water quality. With the introduction of the Resource Management Act in 1991, the preferred treatment for FDE shifted away from traditional two-pond systems to land application. However, on most farms, irrigation of FDE has occurred on a daily basis, often without regard for soil moisture status. Therefore, it has been commonplace for partially treated effluent to drain through and/or runoff soils and contaminate fresh water bodies. The objectives of this thesis were to design and implement a sustainable land application system for FDE on difficult to manage, mole and pipe drained soils, and to assess the impacts of FDE application, urea application and cattle grazing events on nutrient losses via artificial drainage and surface runoff from dairy cattle grazed pasture. To meet these objectives a research field site was established on Massey University's No.4 Dairy farm near Palmerston North. The soil type was Tokomaru silt loam, a Fragiaqualf with poor natural drainage. Eight experimental plots (each 40 x 40 m) were established with two treatments. Four of the plots represented standard farm practice including grazing and fertiliser regimes. Another four plots were subjected to the same farm practices but without the fertiliser application and they were also irrigated with FDE. Each plot had an isolated mole and pipe drainage system. Four surface runoff plots (each 5 m x 10 m) were established as subplots (two on the fertilised plots and two on the plots irrigated with FDE) in the final year of the study. Plots were instrumented to allow the continuous monitoring of drainage and surface runoff and the collection of water samples for nutrient analyses. An application of 25 mm of FDE to a soil with limited soil water deficit - simulating a 'daily' irrigation regime - resulted in considerable drainage of partially treated FDE. Approximately 70% of the applied FDE left the experimental plots with 10 mm of drainage and 8 mm of surface runoff. The resulting concentrations of N and P in drainage and runoff were approximately 45% and 80% of the original concentrations in the applied FDE, respectively. From this single irrigation event, a total of 12.1 kg N ha-1 and 1.9 kg P ha-1 was lost to surface water representing 45% of expected annual N loss and 100% of expected annual P loss. An improved system for applying farm dairy effluent to land called 'deferred irrigation' was successfully developed and implemented at the research site. Deferred irrigation involves the storage of effluent in a two-pond system during periods of small soil moisture deficits and the scheduling of irrigation at times of suitable soil water deficits. Deferred irrigation of FDE all but eliminated direct drainage losses with on average <1 % of the volume of effluent and nutrients applied leaving the experimental plots. Adopting an approach of applying 'little and often' resulted in no drainage and, therefore, zero direct loss of nutrients applied. A modelling exercise, using the APSlM simulation model, was conducted to study the feasibility of practising deferred irrigation at the farm scale on No 4 Dairy farm. Using climate data for the past 30 years, this simulation exercise demonstrated that applying small application depths of FDE, such as 15 mm or less, provided the ability to schedule irrigations earlier in spring and decreased the required effluent storage capacity. A travelling irrigator, commonly used to apply FDE (a rotating irrigator), was found to have 2-3 fold differences in application depth and increased the risk of generating FDE contaminated drainage. New irrigator technology (an oscillating travelling irrigator) provided a more uniform application pattern allowing greater confidence that an irrigation depth less than the soil water deficit could be applied. This allowed a greater volume to be irrigated, whilst avoiding direct drainage of FDE when the soil moisture deficit is low in early spring and late autumn. A recommendation arising from this work is that during this period of low soil water deficits, all irrigators should be set to travel at their fastest speed (lowest application depth) to minimise the potential for direct drainage of partially treated FDE and associated nutrient losses. The average concentrations of N and P in both 2002 and 2003 winter mole and pipe drainage water from grazed dairy pastures were all well above the levels required to prevent aquatic weed growth in fresh water bodies. Total N losses from plots representing standard farm practice were 28 kg N ha-1 and 34 kg N ha-1 for 2003 and 2004, respectively. Total P losses in 2003 and 2004 were 0.35 kg P ha-1 and 0.7 kg P ha-1, respectively. Surface runoff was measured in 2003 and contributed a further 3.0 kg N ha-1and 0.6 kg P ha-1. A number of common dairy farm practices immediately increased the losses of N and P in the artificial drainage water. Recent grazing events increased NO3--N and DIP concentrations in drainage by approximately 5 mg litre-1 and 0.1 mg litre-1, respectively. The duration between the grazing and drainage events influenced the form of N loss due to a likely urine contribution when grazing and drainage coincide, but had little impact on the total quantity of N lost. Nitrogen loss from an early spring application of urea in 2002 was minimal, whilst a mid June application in 2003 resulted in an increased loss of NO3--N throughout 80 mm of cumulative drainage suggesting that careful timing of urea applications in winter is required to prevent unnecessary N leaching. Storage and deferred irrigation of FDE during the lactation season caused no real increase in either the total-N concentrations or total N losses in the winter drainage water of 2002 and 2003. In contrast, land application of FDE using the deferred irrigation system resulted in a gradual increase in total P losses over the 2002 and 2003 winter drainage seasons. However, this increase represents less than 4% of the P applied in FDE during the lactation season. An assessment of likely losses of nutrients at a whole-farm scale suggests that it is standard dairy farming practice (particularly intensive cattle grazing) that is responsible for the great majority of N and P loss at a farm scale. When expressed as a proportion of whole-farm losses, only a very small quantity of N is lost under an improved land treatment technique for FDE such as deferred irrigation. The management of FDE plays a greater role in the likely P loss at a farm scale with a 5% contribution to wholefarm P losses from deferred irrigation.

Irish agriculture is becoming increasingly regulated, with restrictions on fertiliser application rates and stocking rates to reduce nitrate (NO₀⁻) leaching losses. However these regulations have been, to date, based on minimal field research. The purpose of this study was to determine the actual leaching losses of nitrate from Irish dairy pasture at a range of stocking rates, and to investigate the effectiveness of the nitrification inhibitor DCD at reducing nitrate leaching losses where these are deemed excessive. In grazed pastures, a major source of leached nitrate is the urine patch, where a high rate of N is applied in one application. This trial recorded the losses from urine and non-urine areas of pasture separately. Nitrate leaching losses from three soils were recorded using lysimeters at Johnstown Castle, Co. Wexford, over two years. Total nitrate losses were higher from the freely drained Clonakilty and Elton soils than from the heavy Rathangan soil. Mean nitrate losses from urine patches ranged from 16 - 233 kg nitrate-N / ha⁻¹, and were reduced by up to 53% when DCD was applied. DCD also reduced peak and mean nitrate-N concentrations in many cases. In addition, DCD halved the nitrous oxide (N₂O) emission factor on the Rathangan soil, caused increases in pasture N content, and increased herbage yield in some treatments. The distribution of urine patches under dairy grazing was recorded using GPS at Kilworth, Co. Cork. Cows were also found to deposit 0.359 urine patches per grazing hour. A model was produced to predict field-scale nitrate leaching losses from dairy pasture at a range of stocking rates. At 2.94 cows per hectare, the highest stocking rate, annual field N loss was below 34 kg nitrate-N ha⁻¹, mean drainage N concentrations were below 5.65 mg nitrate-N L⁻¹ (the EU drinking water guideline value), and the worst-case-scenario autumn peak concentration did not exceed 21.55 mg nitrate-N L⁻¹ (above the EU Maximum Allowable Concentration (MAC) but below the World Health Organisation (WHO) drinking water limit). DCD reduced total annual field N loss by 21% (a conservative estimate), and also reduced mean and peak nitrate concentrations. Provided fertiliser application rates are at or below 291 kg N ha⁻¹, and based on current legislative values for drinking water quality, this trial does not support any blanket restrictions on the stocking rate of Irish dairy farms. However where particularly high water quality is required, DCD shows potential as a useful tool to achieve low nitrate concentrations.

Worldwide there is considerable concern over the effects of human activities on the quantity and quality of freshwater. Measurement of infiltration behaviour will be important for improving freshwater management. This study identifies that New Zealand has a sporadic history of measuring soil water movement attributes on a limited number of soil types, although the current practical demand should be large for management of irrigation, dairy farm effluent disposal, as well as municipal / domestic waste- and storm-water disposal. Previous research has demonstrated that infiltration behaviour is governed by the interplay between numerous mechanisms including hydrophobicity and preferential flow, the latter being an important mechanism of contaminant leaching for many NZ soils. Future characterisation will need to recognise the dynamic nature of these interactions, and be able to reliably characterise the key infiltration mechanisms. Since macropores are responsible for preferential flow, it is critical that infiltration studies use a representative sample of the macropore network. The aim of this project was to study the mechanisms governing the infiltration behaviour of a layered soil in large (50 x 70 cm) monolith lysimeters, where the connectivity of the macropore network remains undisturbed. Four lysimeters of the Gorge silt loam were collected, a structured soil with four distinct layers. On each lysimeter there were four separate infiltration experiments, with water applied under suctions of 0, 0.5, 1, and 1.5 kPa by a custom-built tension infiltrometer. Each lysimeter was instrumented with 30 tensiometers, located in arrays at the layer boundaries. There was also a field experiment using ponded dye infiltration to visually define preferential flowpaths. Analysis of dye patterns, temporal variability in soil matric potential (Ψm), and solute breakthrough curves all show that preferential flow is an important infiltration mechanism. Preferential flowpaths were activated when Ψm was above -1.5 kPa. During saturated infiltration, at least 97% of drainage was through the ‘mobile’ pore volume of the lysimeter (θm), estimated among the lysimeters at 5.4 – 8.7 % of the lysimeter volume. Early-time infiltration behaviour did not show the classical square-root of time behaviour, indicating sorptivity was not the governing mechanism. This was consistent across the four lysimeters, and during infiltration under different surface imposed suctions. The most likely mechanism restricting sorptivity is weak hydrophobicity, which appears to restrict infiltration for the first 5 – 10 mm of infiltration. Overall, the Gorge soil’s early-time infiltration behaviour is governed by the dynamic interaction between sorptivity, hydrophobicity, the network of air-filled pores, preferential flow and air encapsulation. Long-time infiltration behaviour was intimately linked to the temporal dynamics of Ψm, which was in turn controlled by preferential flow and soil layer interactions. Preferential flowpaths created strong inter-layer connectivity by allowing an irregular wetting front to reach lower layers within 2 – 15 mm of infiltration. Thereafter, layer interactions dominate infiltration for long-time periods, as Ψm in soil layers with different K(Ψm) relationships self-adjusts to try to maintain a constant Darcy velocity. An important finding was that Ψm rarely attained the value set by the tension infiltrometer during unsaturated infiltration. The results show that ‘true’ steady-state infiltration is unlikely to occur in layered soils. A quasi-steady state was identified once the whole column had fully wet and layer interactions had settled to where Ψm changes occurred in unison through each soil layer. Quasi-steady state was difficult to identify from just the cumulative infiltration curve, but more robustly identified as when infiltration matched drainage, and Ψm measurements showed each layer had a stable hydraulic gradient. I conclude that the in-situ hydraulic conductivity, K(Ψm), of individual soil layers can be accurately and meaningfully determined from lysimeter-scale infiltration experiments. My results show that K(Ψm) is different for each soil layer, and that differences are consistent among the four lysimeters. Under saturated flow the subsoil had the lowest conductivity, and was the restricting layer. Most interestingly this pattern reversed during unsaturated flow. As Ψm decreased below -0.5 to -1 kPa, the subsoil was markedly more conductive, and the topsoil layers became the restricting layers. All four soil layers demonstrate a sharp decline in K(Ψm) as Ψm decreases, with a break in slope at ~ -1 kPa indicating the dual-permeability nature of all layers.

New Zealand Agricultural Engineering Institute / Also published as: Agricultural Engineering Thesis no. 4 / For large areas of New Zealand that suffer from agricultural drought, the only practicable way of providing irrigation is through the use of water harvesting schemes that divert winter flood water in nearby streams into off-stream storages for irrigation use in the summer. A community water harvesting scheme is presently under construction in the Glenmark area of North Canterbury which was designed using traditional methods. The objectives of this thesis were to assess the limitations of traditional design methods for water harvesting schemes using the Glenmark Scheme as a case study and to develop an improved method based on a systems modelling approach. A daily simulation model was developed that incorporated in a realistic way the engineering, hydrologic, agronomic and economic features of importance to the design of water harvesting schemes in New Zealand. The model was used to study the adequacy of the traditional methods used for the design of the Glenmark Scheme; to arrive at alternative design solutions that achieved higher levels of engineering, agronomic and economic efficiency; and to develop a better understanding of the nature of complex water harvesting systems. It was demonstrated that compounding conservatism inherent in traditional design methods resulted in scheme overdesign and that the ability of the systems model to capture the essential dynamics of the system allowed higher levels of design performance to be achieved. The experience gained in the use of the systems model led to the development of a formalised design procedure for water harvesting schemes that represents an advance on methods hitherto available.