Ponded Water Depth Research Articles

Effect of Shallow Water Management on Growth and Yield of Rice

Rice, the most staple food crop of the world has been cultivated about 157 million hectares of land (FAO, 2008). Irrigated rice ecosystem contributes 75% of global rice production. Growing rice using conventional irrigation (continuous flooding) requires a tremendous amount of water. The high water demand of irrigated lowland rice mainly arises from keeping the field continuously submerged. There is decreasing trend in water resource availability year by year due to various reasons. The shortage of water resources for rice production has now become an important issue worldwide. It indicates that a reduction in water input without compromising yield and optimization of scarce water in rice production are required. Shallow water management can be one of the alternatives to use water efficiently, achieve good yield and minimize the methane emission from the rice fields. However, information about growth and yield of rice grown under shallow water management are limited. Therefore, this study was conducted to investigate whether shallow water could influence plant growth and yield of irrigated rice under field conditions. Materials and Methods A field experiment was conducted at Yamagata University Experimental Farm, Takasaka, Tsuruoka, Japan in 2009. Two plant spacings – [30*30cm (wider) and 30*15cm (narrow)] and water managements shallow and conventional were practiced .The four treatments were set up as conventional wider (CW), conventional narrow (CN), shallow wider (SW) and shallow narrow (SN). In conventional method, five to six cm water ponding depth was maintained throughout the growing period except for mid season drainage and totally drained out 15 days before the harvest. For shallow water ,ponding water depth of 1-2cm with wetting and drying was done till panicle initiation stage and then 1-2 cm was maintained and totally drained out 30 days before harvesting. Transplanting of 28 days old (3.5leaf age) seedlings of Sasanishiki variety was done on12th May and harvested on 1 st October 2009. The total dry matter & nitrogen (N) uptake by plant was measured at 37, 44, 51, 58, 65, 77, 91 and 122 days after transplanting (DAT). The yield was estimated by both yield components (10hills) and yield examination (50hills).This preliminary experiment was carried without replication. Results and Discussion Comparing with water management practices, aboveground biomass was higher in shallow than conventional. Interestingly, wider spacing of shallow water management showed almost same as to conventional narrow spacing (fig.1).

Response of lowland rice to agronomic management under different hydrological regimes in an inland valley of Ivory Coast

A large proportion of the rice in West Africa is produced in rainfed lowland ecosystems, mainly in inland valleys. The hydrological conditions (duration and intensity of flooding) vary with the toposequence position between the fringe and the centre of the valley. Production methods tend to evolve from the currently predominant unbunded plots without external input use, to input-intensive production in bunded plots. Agronomic management interventions co-evolve and may include varietal choice, herbicide use, and mineral N fertilizer application. The response of rice and the associated weeds to such interventions is likely to vary with the prevailing hydrological regime. A 2-year field experiment was conducted in northern Ivory Coast to determine the impact of water regime (plot position in the valley, presence of bunds) and input use (mineral N fertilizer and herbicide) on the productivity (yield and N use efficiency) of traditional and modern rainfed lowland rice cultivars and the biomass and composition of the associated weeds. Installing field bunds reduced seasonal variations in ponded water depth and resulted in a mean increase in rice grain yield of 30–40% ( p < 0.005). This increase was associated with a 25% lower cumulative weed biomass and a several-fold increase in the agronomic use efficiency of applied mineral N in bunded than unbunded plots. Under low input management, traditional varieties tended to out-yield modern varieties in unbunded plots. Improved crop management such as herbicide and fertilizer application, and the construction of field bunds was more effective to increase the yield and N use efficiency in the flooded valley centre than in the drought-prone valley fringes. There is a need for site-specific targeting of modern cultivars, land development and improved production methods in the inland valleys of the West African savanna zone.

Water losses through paddy bunds: Methods, experimental data, and simulation studies

Summary Irrigated rice landscapes increasingly experience water scarcity, and the bunds surrounding paddy rice fields (also referred to as dikes, banks or levees) have been pointed out as possible causes for low water productivities. Yet, detailed experimental data to confirm this assumption is missing, as well as an identification of the most decisive factors for bund infiltration. The objective of this study was therefore to quantify water fluxes through bunds in a terraced paddy landscape both experimentally and by numerical analysis. Two bunds with a respective age of 20 (young bund: BY) and 100 years (old bund: BO) were investigated at the end of the rice-growing season. Soil core samples revealed a compacted hard pan at 30 cm depth in both bunds. Hydraulic conductivity was very high and variable especially in the upper part of the bunds. For direct measurements of infiltration into the bund, a double-frame infiltrometer was introduced analogous to a double ring infiltrometer. Measured infiltration rates in bund BO at ponding water depths (PWDs) of 3, 5 and 7 cm amounted to 480, 270 and 550 cm2 d−1, respectively, and no cross-flow was observed. At bund BY, the infiltrating water resulted in an increasing water level on the lower field, and the cross-flow was 807, 4205 and 5506 cm2 d−1 at corresponding PWDs. However, no reliable infiltration measurements could be recorded. Tensiometers installed in the bunds during the infiltration experiments revealed a nearly instantaneous rise in pressure head after flooding throughout the profile in bund BY, indicating preferential water fluxes. In bund BO, contrarily, pressure heads underneath the hard pan rose slowly and with a time lag and thus show dominant matrix fluxes. Different infiltration rates in the investigated bunds could be mainly attributed to a lower bulk density and enhanced macroporosity in the young bund BY. The existence of a hard pan in a bund does not necessarily reduce percolation losses, as it might become perforated by macropores originating from bio-activity. A sensitivity analysis realized with HYDRUS 1.05 revealed that under-bund percolation is most strongly influenced by the hydraulic conductivity in the upper and middle bund. Ponding water depth and groundwater table are less important, while soybean cultivation, bund width and elevation difference only play a minor role. For each conductivity profile, there is a critical bund width underneath which cross-flow occurred. A detailed description of the bunds’ water fluxes needs to be included in hydrologic modelling of irrigated rice landscapes.

Modeling of water and nitrogen balance in the ponded water of rice fields

A water and nitrogen balance model for the surface ponded water compartment of rice fields was developed. The model estimates the daily ponded water depth and the daily losses and the uses of NH4–N and NO3–N in their transformation processes. The model was applied with data obtained from two rice fields during 2005 at Thessaloniki plain in northern Greece. Significant amounts of applied irrigation water were lost with the surface runoff and deep percolation to groundwater. The gaseous losses of nitrogen (volatilization and denitrification) and nitrogen uptake by algae were the main processes of nitrogen reduction in the ponded water of rice fields. The study showed that the system of a rice field is a natural system where an important amount of influent nitrogen applied by irrigation water can be reduced. These processes decrease the possibilities of water resources contamination.

Infiltration on alluvial fans in arid environments: Influence of fan morphology

Mountain‐front recharge through highly permeable alluvial fans can be an important source of groundwater recharge in arid climates. To better understand the geomorphic factors (e.g., fan slope, fan area, active channel proportion of fan area, sediment permeability, and entrenchment of the active channel) that control flow and infiltration on alluvial fans, we developed a coupled numerical model of steady surface water flow and Green‐Ampt‐type infiltration. The model was applied to synthetic alluvial fans using random walkers to create connected distributary networks. The purpose of this approach is to predict where and how recharge occurs on fans as a function of fan morphology. Using the numerical model, we examined how the fan shape and the sequence of fan surfaces influenced where infiltration occurred on the fan. We also investigated how fan morphology influenced the partitioning of infiltration between the fan and the valley floor. Finally, we examined how infiltration influenced the spatial distribution of flooding. The greatest amount of infiltration occurred on low gradient fans where water spread laterally with shallower ponded water depths, although the large inundation area often included less permeable sediments outside of the active channel. The ratio of the incision depth to the input flow depth was an important predictor of the amount of infiltration. The greatest amount of infiltration occurred on fans with incision depths slightly smaller than the input flow depth. These results have implications for groundwater resource assessment and for development of monitoring networks on fans in arid environments.

Identifying Risk Factors Affecting Weather- and Disease-Related Losses in the U.S. Farm-Raised Catfish Industry

Two double-limit tobit models are used to identify significant risk factors that most affect farm-raised catfish losses from weather-related events and from disease outbreaks. Results of the weather loss model indicate that the variables for operator education level, number of ponds, pond water depth, production management strategy, past experience with severe losses from low oxygen levels from off-farm power outages, past experience with severe losses from diseases, and being in the South are statistically significant. Results of the disease loss model indicate that the variables for operator experience and pond water depth are significant. Development of models explaining weather and disease losses through observable variables provides a better understanding of the interrelation between the loss perils and explanatory variables so management strategies can be developed to mitigate losses from identified risk factors.

RICEWQ 모형의 보정 및 민감도 분석

Abstract The main objectives of this study are to calibrate the RICEWQ model with Korean field data and then analyse the sensitivity of the parameters to identify sensitive parameters. The RICEWQ is widely used to predict pesticide fate in a paddy plot. An experimental paddy plot of 0.2 ha(100 × 20 m) at Seobyeon-dong, Daegu, Korea was selected, and field observations for water and pesticide balance were performed from 4 June to 2 September 2006. The molinate, which is a herbicide widely used for weed control in rice culture, was selected. The RICEWQ model was successfully calibrated both for the water and pesticide mass balance. The calibrated model showed a RMSE of 0.537 cm for ponded water depths and a RMSE of 0.036 mg/L for the molinate concentrations in the ponded water. The most sensitive parameters for molinate concentrations in ponded water were the metabolism degradation rate in water, volatilization coefficient, and release rate for slow release formulation. In contrast, the RICEWQ model was not sensitive to parameters such as hydrolysis degradation rate in water and degradation rate in unsaturated soil.

The effect of toposequence position on soil properties, hydrology, and yield of rainfed lowland rice in Southeast Asia

A large proportion of rainfed lowland rice in Southeast Asia is grown in gently sloping areas along toposequences with differences in elevation of a few meters. These small differences in elevation can lead to differentiation in soil properties and hydrological conditions, which in turn may affect crop performance and yield. It may be appropriate to replace blanket crop management recommendations in rainfed areas with toposequence-specific management recommendations. However, thorough statistical analyses of the relationships between toposequence position and field and crop conditions are lacking. In this paper, we statistically analysed the effect of toposequence position on soil properties, hydrological conditions, yield, and yield increase due to weed control and/or fertilizer management in rainfed areas in four villages in Indonesia and Thailand each in 2000–2002. Differences were substantial in field hydrology (average depth of ponded surface water and of groundwater, number of days without ponded surface water), exchangeable K, organic C, and clay content depending on toposequence position. There were also differences in other soil properties, including N, P, CEC, pH, sand, silt, bulk density, yield, and the magnitude of yield increase due to intensive weed control and/or recommended fertilizer application, but these effects were not consistent across countries, seasons, and years. The hypothesis that toposequence position would be a useful recommendation domain for weed control and fertilizer recommendations was not supported by our statistical results. The reasons why toposequence position has an inconsistent statistical effect could be (1) that the variability of the field conditions is larger among villages than among toposequence positions, and/or (2) that farmers already respond to differences in field conditions in their prevalent management practices, thus masking the effects of toposequence-specific variation on yield. Our findings suggest that despite the large toposequence effects on soil nutrient and water availability, weed and fertilizer management recommendations should be field-specific and time-specific rather than toposequence-specific.

Development of the HSPF-Paddy model to estimate watershed pollutant loads in paddy farming regions

The Hydrological Simulation Program-FORTRAN (HSPF) was modified to simulate watershed pollutant loads in paddy farming regions. Dike height and the Dirac delta function were added to a modified HSPF code (HSPF-Paddy) to incorporate the behavior of ponded water and nutrients in rice paddy fields. HSPF-Paddy was validated by examining its application to a rice paddy plot (3000 m 2) and a watershed (1803 km 2) that contained a significant proportion of rice paddy fields (30%). The model efficiency at the plot scale was very high showing 0.52, 0.93, and 0.93 for ponded water depth, and accumulation of total nitrogen, and total phosphorus, respectively, and simulated values and observed data were in close agreement. Model efficiency was somewhat lower at the watershed scale showing 0.82, 0.78, 0.44, 0.61, and 0.63 for stream flow, temperature, 5-day biochemical oxygen demand, total nitrogen, and total phosphorus, respectively. However, model performance was generally within expectations, and generally model efficiency greater than 0.5 is deemed acceptable considering the complexity of the watershed in terms of size and land use. This comprehensive, dynamic model is applicable to a wide range of scales, from plot to watershed, and can be used to simulate various paddy field and watershed environmental conditions, including loads from point and nonpoint source pollution. It may be a practical alternative to the unit-load method for estimates of watershed pollution loads, especially in paddy farming regions, where nonpoint source pollution loads may be poorly estimated because of highly site-specific conditions. The application of HSPF-Paddy is recommended for watershed management and best management practices (BMPs) evaluation in paddy farming regions.

Ponding Analysis with Green-and-Ampt Infiltration

The ponding of water over level terrain caused by storms of arbitrary temporal intensity is quantified by solving water-balance differential equations involving cumulative rainfall, infiltration, the depth of ponded water, and evaporation of ponded water. Infiltration is quantified using an extended Green-and-Ampt formulation in which the depth of ponded water influences the infiltration rate. A numerical algorithm for calculating the infiltration and ponding hydrographs is developed and applied in this work. The maximum depth of ponded water and the duration of ponding are among the variables that can be calculated with the method presented in this paper. These variables are useful in basin irrigation and in wetland wetting and restoration.

Exploring options to grow rice using less water in northern China using a modelling approach: I. Field experiments and model evaluation

China's grain basket in the North China Plain is threatened by increasing water scarcity and there is an urgent need to develop water-saving irrigation strategies. Water savings in rice can be realized by alternate wetting and drying (AWD) under lowland conditions, or by aerobic rice in which the crop is grown under nonflooded conditions with supplemental irrigation. Field experimentation and simulation modelling are a powerful combination to understand complex crop-water interactions and to extrapolate site-specific empirical results to other environments and conditions. In this paper, we present results from 4 years of field experiments on AWD and aerobic rice in 2001–2004 near Kaifeng, Henan Province, China. The experimental data were used to parameterize and evaluate the rice growth model ORYZA2000. A subsequent paper reports on the extrapolation of the experimental results using ORYZA2000 and on farmer-participatory testing of aerobic rice. In the lowland area of the study site, rice yields under flooded conditions were around 8000 kg ha −1 with 900 mm total (rain, irrigation) water input. Irrigation water savings were 40–70% without any yield loss by applying AWD. In the upland area of the study site, aerobic rice yielded 2400–3600 kg ha −1, using 750–1100 mm total water input. ORYZA2000 satisfactorily reproduced the dynamics in measured crop variables (biomass, leaf area, N uptake) and soil water variables (ponded water depth, soil water tension). The root mean square error of predicted yield was 11% for lowland rice and 19% for aerobic rice, which was only one and a half times the error in the measured values. We concluded that ORYZA2000 is sufficiently accurate to extrapolate our results on AWD and aerobic rice to different management and environmental conditions in our study area.

Runoff nitrogen from a large sized paddy field during a crop period

Nutrient load management is an important environmental issue because nutrient loads from farmlands degrade surface waters as a result of anthropogenic eutrophication. Nitrogen load from a large sized paddy field during the crop period was examined from the results of field measurements carried out in 2004. The 1.5 ha paddy field was located east of Biwa Lake. Irrigation water volume and ponded water depth were continuously observed. Field measurements were carried out at least once a week to analyze total nitrogen (TN) concentration in the irrigation water and ponded water. Daily inflow and outflow of nitrogen was obtained by multiplication of the nitrogen concentration and transported water volume, consisting of irrigation, precipitation, evapotranspiration, percolation and surface discharge. Water outflow volume was calculated by a tank model that consisted of three small tanks connected to represent ponded water depth differences in the large paddy field. The calculated nitrogen load was 18.8 kg ha −1, with 7.2 kg ha −1 from surface drainage and 11.6 kg ha −1 from percolation loss. The runoff nitrogen value of 18.8 kg ha −1 was within the range of the reported values investigated in a medium-sized paddy field. The observed value was close to the value for a low percolation flux paddy field where less irrigation water has been applied. These results suggest that less irrigation water keep runoff nitrogen low. This also indicates that irrigation water management can reduce nitrogen load from large sized paddy fields.

Variation in plant species richness in and adjacent to 64 ponds in SE Norwegian agricultural landscapes

All vascular plants, classified by life and growth form into six groups, four groups of hydrophytes (lemnids, nymphaeids, elodeids and isoetids), helophytes, and terrestrial species of pond margins, were inventoried in 64 SE Norwegian agricultural landscape ponds and their adjacent margins. The study sites varied considerably with respect to species richness; 0–4 for each hydrophyte group, 0–9 for helophytes, and 13–77 for terrestrial species. A total of 56 explanatory variables were recorded for each pond and adjacent margin to explain the observed richness variability. Species richness in each plant group was modelled separately by generalised linear models (GLM), using the recorded explanatory variables as predictors. Pond area, water depth and water chemical variables were the most important predictors of hydrophyte and helophyte species richness, followed by fluctuation and human impact variables such as liming and presence of ducks and fish. Pond margin area and geographical variables were significant predictors of richness of terrestrial species of pond margins. The contribution of different structuring processes to explanation of species richness patterns is discussed. The generally weak explanatory power of the selected variables indicates high importance of apparent randomness in this ecosystem, and, notably, that the ponds and their adjacent margins represent islands in the agricultural landscape that accumulate species more or less individualistically.

Improvement of application method of liquid fertilizer with irrigation water for uniform topdressing and change in nitrogen concentration of flooded water after application

(pp. 811–816) The application method of liquid fertilizer with irrigation water was tested in large scale paddy fields using a newly developed fertilizer supplier which can make a supplying rate of liquid fertilizer constant. Uniformity of fertilization, change in the nitrogen concentration of flooded water and absorption of applied nitrogen by paddy rice were examined. High uniformity of fertilization was achieved with this method as compared with a conventional broadcast application of granular fertilizer, or an inflow fertilization of granule-like fertilizer with irrigation water. When urea was applied as liquid fertilizer, the fertilizer-N disappeared quickly from ponding water under shallow conditions of ponding water depth. In the case of the same depth of ponding water, the rate of reduction was large under conditions of large permeability of water. The nitrogen utilization rates of urea and ammonium sulfate labeled with 15N were not related to the depth of ponding water at the time of fertilization. When ammonium sulfate was supplied the nitrogen utilization rate was around 50% regardless of fertilization conditions however, the urea-N utilization rate fell to 40% or less at a permeability of ponding water of 0.8 cm day−1 or less, while it increased to 50% equivalent to ammonium sulfate, at 0.8 cm day−1 or more permeability.

Effect of raised beds, irrigation and nitrogen management on growth, water use and yield of rice in south-eastern Australia

To remain economically and environmentally sustainable, Australian rice growers need to be able to readily respond to market opportunities and increase cropping system productivity and water productivity. Water availability is decreasing whereas its price is increasing. Alternative irrigation layouts and water management approaches could contribute to reduced water use and increased irrigation efficiency. This paper reports results for the first crop (rice) in a cropping system experiment to compare permanent raised bed and conventional layouts on a transitional red-brown earth at Coleambally, New South Wales. The performance of conventional ponded rice grown on a flat layout was compared with rice grown on 1.84-m wide, raised beds with furrow and subsurface drip irrigation. In addition, deep and shallow ponded water depth treatments (15 and 5 cm water depth over the beds) were imposed on the rice on beds during the reproductive period. A range of nitrogen (N) fertiliser rates (0–180 kg N/ha) was applied to all treatments. The traditional flat flooded treatment (Flat) achieved the highest grain yield of 12.7 t/ha, followed by the deep (Bed 15) and shallow (Bed 5) ponded beds (10.2 and 10.1 t/ha, respectively). The furrow (Furrow) irrigated bed treatment yielded 9.4 t/ha and the furrow/drip (Furr/Drip) treatment yielded the lowest grain yield (8.3 t/ha). Grain yield from all bed treatments was reduced owing to the wide furrows (0.8 m between edge rows on adjacent beds), which were not planted to rice. Rice crop water use was significantly different between the layout–irrigation treatments. The Flat, Bed 5 and Bed 15 treatments had similar input (irrigation + rainfall – surface drainage) water use (mean of 18.3 ML/ha). The water use for the Furrow treatment was 17.2 ML/ha and for the Furr/Drip treatment, 15.1 ML/ha. Input WP of the Flat treatment (0.68 t/ML) was higher than the raised bed treatments, which were all similar (mean 0.55 t/ML). This single season experiment shows that high yielding rice crops can be successfully grown on raised beds, but when beds are ponded after panicle initiation, there is no water saving compared with rice grown on a conventional flat layout. Preliminary recommendations for the growing of rice on raised beds are that the crop be grown as a flooded crop in a bankless channel layout. This assists with weed control and allows flooding for cold temperature protection, which is necessary with current varieties. Until we find effective herbicides and other methods of weed control and N application that do not require ponding, there is little scope for saving water while maintaining yield on suitable rice soil through the use of beds.

Infiltration under Variable Ponding Depths of Water

Cumulative infiltration was computed as a function of time-varying ponded water depths using a Green and Ampt analysis. The input water depths were field-measured values from two irrigation events on Superstition sand, one from a basin and one from a border. For both types of irrigation, the computed cumulative infiltration at a given measuring station was nearly the same whether using a variable head input or a constant (average) head during the time of opportunity. As an example, at the first measurement station for the basin event, the ponded depth went from a 0 to 9 cm depth over the 2–12 min. from the time water was introduced into the basin; this was followed by a decrease to a depth of 4 cm at 60 min. The computed infiltration using the depth hydrograph was 14.3 cm compared to 14.1 cm when using an average depth. A smaller value of 13.7 cm is found using the appropriate time of opportunity with a field averaged depth and a considerably smaller value of 12.4 cm was found when a zero-depth boundary is considered. Basin and border uniformities were also computed based on variable and different constant depths and the results were found to be reasonably robust whether infiltration is computed using a variable or an appropriate constant ponded depth.

Model development for surface drainage loading estimates from paddy rice fields

A field experiment was performed at two Korean research sites to evaluate water and nutrient behavior in paddy rice culture operations for 2 years. One site was irrigated with groundwater, whereas the other site was irrigated with surface water. Both sites received average annual rainfall of about 1,300 mm, and about 70–80% of it was concentrated during July–September coinciding with rice growing season. Although most of the nutrient outflow was attributed to plant uptake, nutrient loss by surface drainage was substantial. The simplified computer model, PADDIMOD, was developed to simulate water and nutrient behaviors in the paddy rice field. The model predicts daily ponded water depth, surface drainage, and nutrient concentrations. It was formulated with a few equations and simplified assumptions, but its application and a model fitness test indicated that the simulation results reasonably matched the observed data. It is a simple and practical planning model that could be used to evaluate nutrient loading from paddy rice fields alone or in combination with other complex watershed models. Further validation might be required for general application of the PADDIMOD to the simulation of paddy rice fields with various agricultural environments.

Modeling industry-wide sediment oxygen demand and estimation of the contribution of sediment to total respiration in commercial channel catfish ponds

Data collected from 45 commercial channel catfish, Ictalurus punctatus, ponds were used to develop empirical models predicting sediment oxygen demand (SOD). Seven acceptable models were combined with a Monte-Carlo sampling distribution to predict industry-wide sediment oxygen demand (SODi). The SODi values obtained from the best equation were used in simulations to assess the effect of diurnally varying water column dissolved oxygen (DO) concentrations on SOD and the effect of pond water depth on the contribution of SOD to overall pond respiration. Estimated SODi ranged from 62 to 962mgm−2h−1, with a mean of 478mgm−2h−1. There was a 95% probability of mean SODi being ≥700mgm−2h−1. The effects of diurnal variation in DO concentration in the water column on expression of SOD was modeled by combining maximum SODi, an empirical relationship between DO and SOD, and simulated pond DO concentrations. At DO concentrations >15mgl−1, diel SOD in catfish ponds exceeded 20g O2 m−2day−1. But when average diel DO was <4mgl−1 and the range of DO concentration was 6–8mgl−1, SOD decreased to 13g O2 m−2day−1 because DO availability limited the full expression of potential SOD. Respiration totals for sediment (average SODi), plankton, and fish respiration were calculated for pond water depths ranging from 0.25 to 4m. Although whole-pond respiration increases as pond depth increases, the proportion of total respiration represented by sediment decreased from 48 to 10% by increasing water depth over this range. The results of these studies show that SOD is a major component of total pond respiration and that certain management practices can affect the impact of SOD on pond oxygen budgets. Mixing ponds during daylight hours, either mechanically or by orienting ponds for maximum wind fetch, will increase oxygen supply to sediments, thereby allowing maximum expression of SOD and maximum mineralization of sediment organic matter. Given a mixed condition caused by wind or other artificial means, the construction of deeper ponds increases the total mass of DO available for all respiration, causing nighttime DO concentrations to decline at a slower rate, reducing the need for supplemental aeration. Because a pond’s water volume decreases over time from sediment accumulation, annual aeration costs will increase with pond age. Constructing ponds with greater initial depth will therefore reduce long-term cost of aeration, allow more flexible management of pond water budget, and reduce the long-term expense associated with pond reconstruction.

EROSION PROCESSES IN GULLIES MODIFIED BY ESTABLISHING GRASS HEDGES

Concentrated flow can cause gully formation on sloping lands and in riparian zones of floodplains adjacent to incising stream channels. Current practice for riparian gully control involves blocking the gully with an earthen embankment and installing a pipe outlet. Measures involving native vegetation would be more attractive for habitat recovery and economic reasons. To test the hypothesis that switchgrass (Panicum virgatum L.) hedges planted at 0.5 m vertical intervals within a gully would control erosion, we established a series of hedges in several concentrated flow channels. Two of the channels were previously eroded trapezoidal channels cut into compacted fill in an outdoor laboratory. The other channels were located at the margin of floodplain fields adjacent to an incised stream channel (Little Topashaw Creek) in Chickasaw County, Mississippi. While vegetation was dormant following two growing seasons, we created artificial runoff events in our test gullies using synthetic trapezoidal-shaped hydrographs with peak discharge rates of approximately 0.03, 0.07, and 0.16 m3 s-1, flow rates similar to those observed during natural runoff events in gullies at Topashaw. During these tests, we monitored flow depth, velocity, turbidity, and soil pore water pressures. Flow depths were generally <0.3 m, and flow velocities varied spatially and exceeded 2.0 m s-1 at the steepest points in some tests. Erosion rates remained modest for the conditions tested, as long as slopes were less than 3 horizontal to 1 vertical (33%) and step height between hedges was less than 0.5 m. Stability modeling of soil steps reinforced with switchgrass roots showed that cohesive forces were 3 times greater than shearing forces for 0.5 m step heights, and that therefore mass failure was unlikely even with the surcharge weight of a 0.2 m depth of ponded water. For step heights greater than 1 m, however, mass failure was observed and predicted to be the dominant erosion mechanism.

Model Development for Nutrient Loading Estimates from Paddy Rice Fields in Korea

A field experiment was performed to evaluate water and nutrient balances in paddy rice culture operations during 2001–2002. The water balance analysis indicated that about half (50–60%) of the total outflow was lost by surface drainage, with the remainder occurring by evapotranspiration (490–530 mm). The surface drainage from paddy fields was mainly caused by rainfall and forced-drainage, and in particular, the runoff during early rice culture periods depends more on the forced-drainage due to fertilization practices. Most of the total phosphorus (T-P) inflow was supplied by fertilization at transplanting, while the total nitrogen (T-N) inflow was supplied by the three fertilizations, precipitation, and from the upper paddy field, which comprised 13–33% of the total inflow. Although most of the nutrient outflow was attributed to plant uptake, nutrient loss by surface drainage was substantial, comprising 20% for T-N and 10% for T-P. Water and nutrient balances indicate that reduction of surface drainage from paddy rice fields is imperative for nonpoint source pollution control. The simplified computer model, PADDIMOD, was developed to simulate water and nutrient (T-N and T-P) behavior in the paddy rice field. The model predicts daily ponded water depth, surface drainage, and nutrient concentrations. It was formulated with a few equations and simplified assumptions, but its application and a model fitness test indicated that the simulation results reasonably matched the observed data. It is a simple and convenient planning model that could be used to evaluate BMPs of paddy rice fields alone or in combination with other complex watershed models. Application of the PADDIMOD to other paddy rice fields with different agricultural environments might require further calibration and validation.