Typewritten copy Includes bibliographical references The measurement of soil moisture in situ is called Part A; The study of soil moisture changes under a simple crop rotation is called Part B. Evaluates the Bouyoucos electrical resistance method of measuring soil moisture in situ; and studies the soil moisture under a simple crop rotation. Has been the subject of 2 papers entiled: Techniques associated with the use of gypsum block soil moisture meters / Aitchinson, Butler and Gurr; and, Gypsum block moisture meters as instruments for the measurement of tension in soil water / Aitchinson and Butler. The papers are attached. Part A includes a brief summary and discussion of these papers

Unsaturated hydraulic conductivity was measured using four different methods. Tension permeameters were used to measure unsaturated hydraulic conductivity in the field, using a single disc method, which depends on the measurements of sorptivity, steady state flow rate, initial and final water content (White and Perroux, 1987, 1989). Also, a double disc method was used which utilizes Wooding's (1968) equation for two different disc radii at the same tension for steady state flow rates. Undisturbed and disturbed soil cores were used to measure unsaturated hydraulic conductivity in the lab, using water retention curves with van Genuchten's equations. There were no significant differences in the mean of hydraulic conductivity between single and double disc methods in all the tensions used (0, 5, 10 and 15 cm). There were significant differences between the field methods and undisturbed soil cores in zero cm tension, and disturbed soil cores in 10 and 15 cm tension. The effect of land preparation on the unsaturated hydraulic conductivity was studied using the double disc method. Tilling has significant effects on the unsaturated hydraulic conductivity at all tensions used. The spatial variation of unsaturated hydraulic conductivity and steady state flow in different tensions using the double disc method was studied. We found exponential variogram models for unsaturated hydraulic conductivity at 5, 10 and 15 cm tensions and a random model for zero cm tension. Also, exponential models were best fitted for steady state flow corresponding to pores radii of 0.03 - 0.015 cm, 0.015 - 0.010 cm and steady state flow at 10 cm tension. A Michaelis-Menton model was used for steady state flow at 5 cm and 15 cm tension. Disc permeameters were also used to add 5 cm depth of water, bromide and dye solution at 0, 5, 10 and 15 cm tensions with three replicates. A comparison was made between field data and simulated model under the same boundary and initial conditions as in the field. Results showed that the water and bromide move deeper than the prediction of the simulated model in all tensions used. The differences were larger between simulated model and field data for both water and bromide concentrations in the lower tension and smaller in the higher tension as a result of elimination of some preferential flow paths. An equation was developed for cumulative infiltration valid for both small and large time. The parameters calculated using the developed equation closely matched the measured infiltration, and fit better than a three term series similar to the Philip equation for one-dimensional flow.

The feasibility of synthesizing distributed fields of remotely-sensed soil moisture by the novel application of four-dimensional data assimilation applied in a hydrological model was explored in this study. Six Push Broom Microwave Radiometer images gathered over Walnut Gulch, Arizona were assimilated into the TOPLATS hydrological model. Several alternative assimilation procedures were implemented, including a method that adjusted the statistics of the modeled field to match those in the remotely sensed image, and the more sophisticated, traditional methods of statistical interpolation and Newtonian nudging. The high observation density characteristic of remotely-sensed imagery poses a massive computational burden when used with statistical interpolation, necessitating observation reduction through subsampling or averaging. For Newtonian nudging, the high observation density compromises the conventional weighting assumptions, requiring modified weighting procedures. Remotely-sensed soil moisture images were found to contain horizontal correlations that change with time and have length scales of several tens of kilometers, presumably because they are dependent on antecedent precipitation patterns. Such correlation therefore has a horizontal length scale beyond the remotely sensed region that approaches or exceeds the catchment scale. This suggests that remotely-sensed information can be advected beyond the image area and across the whole catchment. The remotely-sensed data was available for a short period providing limited opportunity to investigate the effectiveness of surface-subsurface coupling provided by alternative assimilation procedures. Surface observations were advected into the subsurface using incomplete knowledge of the surface-subsurface correlation measured at only 2 sites. It is perceived that improved vertical correlation specification will be a need for optimal soil moisture assimilation. Based on direct measurement comparisons and the plausibility of synthetic soil moisture patterns, Newtonian nudging assimilation procedures were preferred because they preserved the observed patterns within the sampled region, while also calculating plausible patterns in unmeasured regions. Statistical interpolation reduced to the trivial limit of direct data insertion in the sampled region and gave less plausible patterns outside this region. Matching the statistics of the modeled fields to those observed provided plausible patterns, but the observed patterns within sampled area were largely lost.

Water management for the irrigation of containerized production remains a task reserved to a highly qualified personnel. Without proper instruments to determine the actual water needs, nursery crops can face growth losses of 20 to 30%. / Reviewing the different methods that are used in field production, tensiometer have shown the greatest potential of adaptation to potted plants. / Tests on the utilization of tensiometer were realized on two peat-vermiculite substrates, using two types of porous cup (1 bat--standard flow and 1 bar--high flow). The instruments were tested while permanently fixed in the growing medium, and temporarily fixed for three hours in the growing medium. / The results show that the instruments give excellent measurements for normal condition of growth, requiring saturation level between 50 and 100%, and that for both mode of operation. / Preferably, the choice of the instrument should be based on the hydraulic characteristics of the substrate. Little difference has been found using both instrument in the substrate 1, but in substrate 2 however, the high flow porous cups have given better response. The high flow cups have higher porosity and bigger pores that suit better substrate 2, which is made of coarser particles.

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Typescript. / Thesis (Ph. D.)--University of Hawaii at Manoa, 1984. / Bibliography: leaves [187]-199. / Microfiche. / xvii, 199 leaves, bound ill. 29 cm

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Water is a scarce resource used by various stakeholders. Agriculture is one of the users of this resource especially for growing plants. Plants need to take up carbon dioxide to prepare their own food. For this purpose plants have stomatal openings. These same openings are used for transpiration. Quantifying transpiration is important for efficient water resource management and crop production because it is closely related to dry matter production. Transpiration could be measured using a number of methods or calculated indirectly through quantification of the soil water balance components using environmental instruments. The use of models such as the Decision Support System for Agrotechnology Transfer (DSSAT v3.5) is, however, much easier than environmental instruments. Nowadays, with increased capabilities of computers, the use of crop simulation modelling has become a common practice for various applications. But it is important that models, such as DSSAT v3.5, be calibrated and verified before being used for various applications such as long-term risk assessment, evaluation of cultural practices and other applications. In this study the model inputs have been collected first Then the model was calibrated and verified. Next sensitivitY analysis was carried to observe the model behavior to changes in inputs. Finally the model has been applied for long-term risk assessment and evaluation of cultural practices. In this study, the data collected formed the basis forthe minimum dataset needed for running the DSSAT v3.5 model. In addition, the factory given transmission of shading material over a tomato crop was compared to actual measurements. Missing weather data (solar irradiance, minimum and maximum air temperature and rainfall) were completed after checking that it was homogeneous to measurements from nearby automatic weather station. It was found that factory-given transmission value of 0.7 of the shade cloth was different from the actual one of 0.765. So this value was used for conversion of solar irradiance measured outside the shade cloth to solar irradiance inside the shade cloth. Conventional laboratory procedures were used for the analysis of soil physical and chemical properties. Soil water content limits were determined using texture and bulk density regression based equations. Other model inputs were calculated using the DSSAT model. Crop management inputs were also documented for creation of the experimental details file. The DSSATv3.5 soil water balance model was calibrated for soil, plant and weather conditions at Ukulinga by modifying some of its inputs and then simulations of the soil water balance components were evaluated against actual measurements. For this purpose half of the data available was used for calibration and the other half for verification. Model simulations of soil water content (150 to 300 mm and 450 to 600 mm) improved significantly after calibration. In addition, simulations of leaf area index (LA!) were satisfactory. Simulated evapotranspiration (E1) had certain deviations from the measured ET because the latter calculated ET by multiplying the potential ET with constant crop multiplier so-called the crop coefficient. Sensitivity analysis and long-term risk assessments for yield, runoff and drainage and other model outputs were carried out for soil, plant and weather conditions at Ukulinga. For this purpose, some of the input parameters were varied individually to determine the effect on seven model output parameters. In addition, long-term weather data was used to simulate yield, biomass at harvest, runoff and drainage for various initial soil water content values. The sensitivity analysis gave results that conform to the current understanding of the soil-plant atmosphere system. The long-term assessment showed that it is risky to grow tomatoes during the winter season at Ukulinga irrespective of the initial soil water content unless certain measures are taken such as the use of mulching to protect the plants from frost. The CROPGRO-Soya bean model was used to evaluate the soil water balance and gro'W1:h routines for soil, plant and weather conditions at Cedara. In addition, cultural practices such as row spacing, seeding rate and cultivars were also evaluated using longterm weather data. Simulations of soil water content were unsatisfactory even after calibration of some of the model parameters. Other model parameters such as LAI, yield and flowering date had satisfactory agreement with observed values. Results from this study suggest that the model is sensitive to weather and cultural practices such as seeding rates, row spacing and cultivar maturity groups. The general use of decision support systems is limited by various factors. Some of the factors are: unclear definition of clients/end users; no end user input prior to or during the development of the DSS; DSS does not solve the problems that the client is experiencing; DSS do not match their decision-making style; producers see no reason to change the current management practices; DSS does not provide benefit over current decision-making system; limited computer ownership amongst producers; lack of field testing; producers do not trust the output due to the lack of understanding of the underlying theories of the models utilized; cannot access the necessary data inputs; lack of technical support; lack of training in the development ofDSS software; marketing and support constraints; institutional resistances; short shelf-life of DSS software; technical constraints, user constraints and other constraints. For successful use of DSS, the abovementioned constraints have to be solved before their useful impacts on farming systems could be realized. This study has shown that the DSSAT v3.5 model simulations of the soil water balance components such as evapotranspiration and soil water content were unsatisfactory while simulations of plant parameters such as leaf area index, yield and phonological stages were simulate to a satisfactory standard. Sensitivity analysis gave results that conform to the current understanding of the soil-plant -atmosphere system. Model outputs such as yield and phonological stages were found to sensitive to weather and cultural practices such as seeding rates, row spacing and cultivar maturity groups. It ha been further investigated that the model could be used for risk assessment in various crop management practices and evaluation of cultural practices. However, before farmers can use DSSAT v3.5, several constraints have to be solved. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 2003.

Spatial variation of in situ available water content was studied along with related parameters over three 16-ha irrigated fields. The fields, two near Marana (Pima County, Arizona) and one near Casa Grande (Pillai County), range in texture from very fine loam to loamy sand. All soil series present are mapped as Entisols or Aridisols. A 50-m grid provided 56 sampling sites in each field. Each site was sampled at 30, 60, 90, 120 and 150 cm. Samples were collected from each field following a heavy irrigation in March 1977. Bulk samples were collected two days and four weeks after the application of approximately 300 mm of water, to determine in situ water content at field capacity and moisture redistribution with time. Related parameters such as particle size distribution and soil water characteristics were also studied. Bulk density and saturated hydraulic conductivity were determined from undisturbed, core samples. The measured parameters showed different patterns of variation within the same field as well as from one field to the other. Spatial variability of saturated hydraulic conductivity was the highest for which coefficient of variability (CV) ranged upward to 108%. Bulk density, on the other hand, showed the lowest coefficient of variability, as low as 5%. The in situ available water content (AWC), estimated by subtracting moisture content at 15 bars from the corresponding in situ FC values, showed a general tendency to increase with depth corresponding to the increase in percent silt plus sand with depth in all three fields. The coefficient of correlation between the two parameters was high (up to 0.70). The mean values of AWC as estimated using 0.1 bar values for field capacity in the laboratory were consistently higher than the in situ values. The values were within 25 - 35% of each other in Fields 1 and 2, while in the sandier soil of Field 3, the AWC was overestimated by an average of 74% in the laboratory. The CV showed an irregular tendency to increase with depth, but was consistently high in the 150 cm layer in all three fields. Values estimated in the laboratory showed lower CV and higher correlations with soil separates than in situ AWC in all three fields. These two observations can be attributed to the elimination of in situ factors such as texture stratification, compaction, and/or amount of water applied. Agricultural soil formed on water transported material at 0.1 bar were highly correlated with sand (r = -0.8) and the 15 bar values were better correlated with clay (r = 0.5). Also, the coefficient of variability increased consistently with decreases in moisture content. The analysis of variance showed the three fields to be heterogeneous. The variation for within and between the 5 depth groups was significant. A two-way interaction between depths and subareas within each field accounted for 44, 45 and 38% of the total variability in Fields 1, 2, and 3 respectively. Cumulative frequency distribution plots, full normal plots, Kolmogorov-Smirnov tests of goodness-of-fit, tests of skewness and tests of kurtosis were conducted to test the null hypothesis of normal distribution for each parameter. The full normal plots, being sensitive to deviations from normality, rejected the null hypothesis in all cases with few exceptions. They showed the data tends to be skewed to the right and/or kurtic. The alternative frequency distribution of the parameters indicated the data to be asymmetric, short tailed with the exception of percent sand which was symmetric, short tailed for all three fields. A power transformation is suggested as a possibility for transforming the data to get near normal distribution.

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