Current irrigation scheduling technologies are limited to refilling the root zone based on measured or predicted amount of water stored within the root zone. This needs measurement of soil-water status and specifying soil field capacity that make this approach expensive and challenging. The FullStopTM wetting front detector (FS) was specifically developed to be a simple and affordable technology to help farmers manage water, nutrients and salts in the root zone. This device responds to a strong wetting front, but research has shown it is less sensitive to weak redistributing wetting fronts, and this may compromise its efficacy in certain situations. The objectives of this study were to recommend a modified version of the FS that responds to weak redistributing wetting fronts and to develop guidelines for the deployment of these detectors to schedule irrigation. The research described herein comprises of two phases: the first phase focused on literature review, field evaluation of wetting front detector of varying sensitivities (WFD) and laboratory measurements of hydraulic properties of soil and wick materials. The second phase validates the HYDRUS-2D/3D for the development of guidelines on how to use WFD to schedule irrigation. The first phase includes: i) a literature review on passive lysimetry that relates design features to the sensitivity of WFD and how prototypes of WFD operate; ii) hydraulic characterization of soil and wick materials to describe the functioning of the different WFD designs; iii) an empirical investigation to determine whether the wick characteristics limits the attainment of equilibrium between the opening of the outer tube and the water table in the inner tube; iv) field evaluations of five types of WFD under sprinkler and natural rainfall to examine the accuracy and sensitivity of the different WFD designs; and v) analysis of the equilibrium between the WFD and the surrounding soil, and recommendations for the best design options based on the sensitivity requirement for different situations. The second phase of the study used observed data sets to validate the Hydrus-2D/3D model. After validation, the model was used to simulate different irrigation scenarios to develop guidelines for the deployment of WFD to schedule irrigation. Field evaluations of various WFD designs showed that length has significant effect on the sensitivity of WFD (P ≤ 0.05). The 90-cm-long Tube wetting front detector (90TD) was significantly more sensitive than the original FS design. The hydraulic conductivity function of two wick materials (Diatomaceous Earth and Fine sand) were not limiting for the attainment of the equilibrium between the Tube Detector and the surrounding soil, and the opening of the Tube Detector and the water level in the inner tube. The Hydrus-2D/3D model performed well in simulating the measured responses of FS or 90TD and the experimental sensitivity thresholds of these detectors. This model was deployed to link WFD responses to different simulated irrigation scenarios to generate monitoring protocol such as detector placement depth, irrigation amount or interval. The model simulations showed that FS can be used to schedule irrigation objectively for sprinkler or drip irrigations, i.e. adjusting irrigation amount or interval based on the response of a detector. Though further study is warranted, model simulation has indicated that 90TD can be used to improving furrow irrigation management. It is envisaged that WFD technology can guide farmers to make informed irrigation decisions and alerting farmers to percolation losses below the root zone. / Thesis (PhD)--University of Pretoria, 2014 / gm2015 / Plant Production and Soil Science / PhD / Unrestricted

Irrigation needs to be applied to soils in relatively humid regions such as western Kentucky to supply water for crop uptake to optimize and stabilize yields. Characterization of soil and crop variability at the field scale is needed to apply site specific management and to optimize water application. The objective of this work is to propose a characterization and modeling of soil and crop processes to improve irrigation management. Through an analysis of spatial and temporal behavior of soil and crop variables the variability in the field was identified. Integrative analysis of soil, crop, proximal and remote sensing data was utilized. A set of direct and indirect measurements that included soil texture, electrical conductivity (EC), soil chemical properties (pH, organic matter, N, P, K, Ca, Mg and Zn), NDVI, topographic variables, were measured in a silty loam soil near Princeton, Kentucky. Maps of measured properties were developed using kriging, and cokriging. Different approaches and two cluster methods (FANNY and CLARA) with selected variables were applied to identify management zones. Optimal scenarios were achieved with dividing the entire field into 2 or 3 areas. Spatial variability in the field is strongly influenced by topography and clay content. Using Root Zone Water Quality Model 2.0 (RZWQM), soil water tension was modeled and predicted at different zones based on the previous delineated zones. Soil water tension was measured at three depths (20, 40 and 60 cm) during different seasons (20016 and 2017) under wheat and corn. Temporal variations in soil water were driven mainly by precipitation but the behavior is different among management zones. The zone with higher clay content tends to dry out faster between rainfall events and reveals higher fluctuations in water tension even at greater depth. The other zones are more stable at the lower depth and share more similarities in their cyclic patterns. The model predictions were satisfactory in the surface layer but the accuracy decreased in deeper layers. A study of clay mineralogy was performed to explore field spatial differences based on the map classification. kaolinite, vermiculite, HIV and smectite are among the identified minerals. The clayey area presents higher quantity of some of the clay minerals. All these results show the ability to identify and characterize the field spatial variability, combining easily obtainable data under realistic farm conditions. This information can be utilized to manage resources more effectively through site specific application.

Thesis (MScAgric)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: Irrigation scheduling decisions are based either on the direct measurement of soil water status (SWS) or on physiological measurements like plant water status (PWS). Soil based measurements are quick and easily automated, but the plant response for a particular quantity of soil moisture varies as a complex function of evaporative demand. A plant-based approach measures the plant stress response directly, but is an integration of environmental effects as well. In contrary to soil-based methods, plant based measurements can indicate when to irrigate, but not the quantity. Pre-dawn leaf water potential (ΨPD) is determined mostly by the soil moisture level, and can serve as a measure of static water stress in plants and an index of bulk soil water availability or even as an estimate of soil water potential at the root surface. Therefore it should be possible to establish a link between SWS and PWS, but it is largely unknown how stable the link in a heterogeneous vineyard would be, and how the grapevine vegetative and reproductive response relates to this link. Plant water status plays a large role in determining vigour and yield of the plant. The levels of PWS are influenced by irrigation, but it was mostly affected by the season and vine location in the vineyard. More negative plant water potentials reduced vigour, but had a less pronounced effect on yield, while also reducing overall wine quality. Vigour variability in the vineyard was largely attributed to soil heterogeneity, which seemed to have a strong effect on SWS. SWS measurements were calibrated according to the observed variability, increasing the accuracy of measurements significantly. Soil water content values were used to establish a link between SWS and PWS. This link was determined over time using nine plots, consisting of rain-fed and irrigated regimes, in variable vigour areas. A non-linear relationship was found between ΨPD and percentage extraction of plant available water for rain-fed plots. When irrigation was applied, no correlation could be found. In this study, for Merlot in the Stellenbosch region, PWS differences affected vigour, and to a lesser extent yield, as well as wine quality. More negative plant water potentials reduced vigour more in high vigour areas than in lower vigour areas, which in turn led to unbalanced vegetative: reproductive ratios. This disturbed vine balance may have had a bigger impact on wine quality than PWS levels. Therefore a well-managed and balanced vine is able to withstand more stress, with less detrimental effects. This study also highlights the danger of limiting the assessment of soil and plant water status conditions to point measurements in vineyards with high levels of vigour variability. / AFRIKAANSE OPSOMMING: Besluite rakende die skedulering van besproeiing word gewoonlik gebaseer op die direkte meting van grondwaterstatus (GWS), of op fisiologiese metings soos byvoorbeeld plantwaterstatus (PWS). Grond gebaseerde metings is relatief vinnig en maklik om te outomatiseer, maar die plantrespons vir ʼn spesifieke grondwaterinhoud varieer as ʼn komplekse funksie van dampdruktekorte. ‘n Plantgebaseerde benadering meet die plantstresreaksie direk, maar is ʼn integrasie van omgewingstoestande. In teenstelling met grondgebaseerde metodes, kan plantgebaseerde metodes aandui wanneer om te besproei, maar nie die hoeveelhede wat besproei moet word nie. Voorsonsopkoms blaarwaterpotensiaal (ΨPD) word grootliks deur die grondwaterinhoud bepaal, en kan as ʼn maatstaf van statiese waterspanning in plante en as ʼn indeks van bulk grondwaterbeskikbaarheid dien, of selfs as ʼn benadering van die grondwaterpotensiaal by die worteloppervlak. Dit behoort dus moontlik te wees om ʼn verwantskap te bepaal tussen GWS en PWS, maar dit is grootliks onbekend hoe stabiel hierdie verwantskap sal wees in ʼn heterogene wingerd, asook hoe die wingerdstok se vegetatiewe en reproduktiewe reaksie die verwantskap kan beïnvloed. Plantwaterstatus speel ʼn groot rol in die bepaling van groeikrag en opbrengs in die wingerdstok. Die vlakke van plantwaterstatus word deur besproeiing beïnvloed, maar word skynbaar meesal deur die seisoen en wingerdstok se ligging in die wingerd bepaal. Meer negatiewe plantwaterpotensiaalvlakke het gelei tot laer groeikrag, maar het ʼn minder uitgesproke effek gehad op opbrengs, terwyl dit in die algemeen wynkwaliteit verswak het. Groeikrag variasie in die wingerd kon grootliks aan grond heterogeniteit toegeskryf word, wat skynbaar ʼn sterk invloed op grondwaterstatus gehad het. Grondwaterstatus metings is gekalibreer volgens die variasie wat waargeneem is, wat die akkuraatheid van metings beduidend verhoog het. Grondwaterinhoud waardes is gebruik om ʼn verwantskap aan te dui tussen SWS en PWS. Hierdie verwantskap is oor tyd bepaal vir nege persele, wat bestaan het uit droëland asook besproeide persele, in areas waarvan die groeikrag verskil het. ʼn Nie-liniêre verband is gevind tussen ΨPD en die persentasie onttrekking van plantbeskikbare water vir die droëland persele. Waar besproei was, kon geen verband gevind word nie. In hierdie studie, vir Merlot in die Stellenbosch area, het PWS vlakke groeikrag en tot ʼn mindere mate opbrengs en wynkwaliteit beïnvloed. Meer negatiewe plantwaterpotensiaal vlakke het groeikrag meer beïnvloed in hoër groeikrag areas as in die laer groeikrag areas, wat ook gelei het tot ongebalanseerde vegetatiewe:reproduktiewe verhoudings. Hierdie versteurde balans in die wingerdstokke kon dalk ʼn groter impak op wynkwaliteit gehad het as PWS vlakke. Daar moet dus gepoog word om goed bestuurde en gebalanseerde wingerdstokke te hê, sodat strestoestande beter weerstaan kan word met minder nadelige gevolge. Die studie beklemtoon ook die gevaar verbonde daaraan om die bepaling van grond- en plantwaterstatus te beperk tot puntmetings in wingerde met groot variasie in groeikrag.