Development and application of a modeling approach for surface water and groundwater interaction

Investigation of the interaction of surface water (SW) and groundwater (GW) is critical in order to determine the effects of best management practices (BMPs) on the entire system of water resources. The objective of this research was to develop a modeling system for considering SW–GW interactions and to demonstrate the applicability of the developed system. A linked modeling approach was selected to consider SW–GW interaction. The dual-simulation scheme was developed to consider different time scales between a newly developed surface model: Dynamic Agricultural Non-point Source Assessment Tool (DANSAT), and existing groundwater models; a three-dimensional finite-difference groundwater flow model (MODFLOW) and a modular three-dimensional transport model (MT3D). A distributed and physically based DANSAT predicts the movement of water and pesticides in runoff and in leachate at a watershed scale. MODFLOW and MT3D simulate groundwater and pesticide movement in the saturated zone. Only the hydrology component of the linked system was evaluated on the QN2 subwatershed in the Nomini Creek watershed located in the Coastal Plain of Virginia mainly due to lack of observed data for MT3D calibration. The same spatial scale was used for both surface and groundwater models while different time scales were used because surface runoff occurs more quickly than groundwater flow. DANSAT and MODFLOW were separately calibrated using the integrated GW approach which uses own lumped baseflow components in DANSAT, and using the steady-state mode in MODFLOW, respectively. Then the linked system was applied to QN2 based on the parameters selected for DANSAT and MODFLOW to simulate time-dependent interactions on the entire system. The linked approach was better than the integrated approach for predicting the temporal trends of monthly runoff by improving the monthly Nash–Sutcliffe efficiency index from 0.53 to 0.60. The proposed linked approach will be useful for evaluating the impacts of agricultural BMPs on the entire SW–GW system by providing spatial distribution and temporal changes in groundwater table elevation and enhancing the reliability of calibrated parameter sets.     

Exploring regional irrigation water demand using typologies of farms and production units: An example from Tunisia

Most methods used to predict irrigation water consumption at a regional scale are based on biophysical models and cropping patterns. Their aim is to provide accurate estimations of “water demand” that are useful for water resource management. However, in the case of free access to the water resource, for example pumping from a water table, it is only possible to prevent overexploitation by “managing” the demand for water, which thus needs to focus on farmers’ choices and behavior. In this paper, we propose a framework to represent agricultural activities using typologies of farms and production units aggregated at a regional scale. The framework can be used to estimate consumption of irrigation water and of other inputs, as well as the production of outputs. The framework can also be used to evaluate the effects of technical, economic or institutional changes on farm income, and to predict the consequences of changes for farmers’ choices at regional scale. We used this method in Central Tunisia to estimate irrigation water demand in 1999. We then simulated the changes that would occur if drip irrigation were adopted. The results of the simulation showed some savings in water and in labor, and, with fertigation, an increase in yields. Using drip irrigation would consequently enable farmers to extend the area of drip-irrigated land. We then simulated the widespread adoption of drip irrigation and the resulting extension of irrigated areas: the results showed no savings in water at the regional scale. These hypotheses were confirmed in 2005 using new typologies to estimate the new demand for irrigation water. We also simulated the effects of economic changes on farm incomes. A major increase in the cost of water affected a minority of farms, which consumed only 17% of total irrigation water, whereas a slight decrease in watermelon and melon prices affected a majority of farms, which consumed 78% of total irrigation water. Water demand management tools therefore need to focus on the effects of technical, economic, or institutional changes and on farmers’ choices.     

A GIS-based model to estimate the regionally distributed drought water demand

A GIS-based Water Resources and Agricultural Permitting and Planning System (GWRAPPS) was developed by integrating the Agricultural Field Scale Irrigation Requirements Simulation (AFSIRS) crop water model, a geographic information system (GIS) and a database management system within an ArcGIS framework. GWRAPPS facilitates the quantification of irrigation water for regional planning and farm scale permitting purposes under statistically average to drought conditions using spatially distributed soils, land-use, and long-term daily climate data. In addition, the system provides regional estimates of daily water withdrawals that are necessary for input into conjunctive surface/groundwater models. This paper presents two Florida case studies that demonstrate GWRAPPS’ ability to characterize irrigation needs based on spatially heterogeneous soil and climate data in contrast to a spatially lumped model. The results show that while inclusion of soil heterogeneity is important to capture water requirements at individual farms, regional water demands are adequately captured using each farm’s predominant soil.     

Surface energy balance model of transpiration from variable canopy cover and evaporation from residue-covered or bare-soil systems

A surface energy balance model based on the Shuttleworth and Wallace (Q J R Meteorol Soc 111:839–855, 1985) and Choudhury and Monteith (Q J R Meteorol Soc 114:373–398, 1988) methods was developed to estimate evaporation from soil and crop residue, and transpiration from crop canopies. The model describes the energy balance and flux resistances for vegetated and residue-covered surfaces. The model estimates latent, sensible and soil heat fluxes to provide a method to partition evapotranspiration (ET) into soil/residue evaporation and plant transpiration. This facilitates estimates of the effect of residue on ET and consequently on water balance studies, and allows for simulation of ET during periods of crop dormancy. ET estimated with the model agreed favorably with eddy covariance flux measurements from an irrigated maize field and accurately simulated diurnal variations and hourly amounts of ET during periods with a range of crop canopy covers. For hourly estimations, the root mean square error was 41.4 W m⁻², the mean absolute error was 29.9 W m⁻², the Nash–Sutcliffe coefficient was 0.92 and the index of agreement was 0.97.     

Sampling strategies for soil water content to estimate evapotranspiration

When the soil water balance method is applied at a field scale, estimation of the spatial variability and confidence interval of actual evapotranspiration is rare, although this method is sensitive to the spatial variability of the soil, and thus to the sampling strategy. This work evaluated the effect of soil sampling strategies for soil water content and water flux at the bottom of the soil profile on the estimation of the daily and cumulative evapotranspirations. To do that, according to the statistical properties of daily measurements in a field experiment with a soybean crop, the water content and flux through the base to the soil profile in space (field scale) and time (daily scale) were simulated. Four different sampling strategies were then compared, and their effects on daily and seasonal cumulative evapotranspirations quantified. Strategy 1 used ten theoretical sites randomly located in the field. The daily water content estimates were assumed to be available each day from these same ten locations, which were located from 0.15 m to 1.55 m in depth, with space steps of 0.10 m. Strategy 2 assumed that daily water content estimates combined two sources: in the 0.00–0.20 m soil layer, ten theoretical sites were selected but changed every day, with thin soil layers for soil moisture sampling, from 1 to 5 cm in thickness. In the 0.20–1.60 m soil layer, the daily water content estimates were assumed to come from the same ten locations (the first soil moisture estimate was located at 0.25 m, and the others were located every 0.10 m until 1.55 m). Strategy 3 used ten theoretical sites located in the field, as in strategy 1, however the water content estimates in the 0.00–0.20-m soil layer were assumed to come from accurate water content measurements (soil layers from 1 to 5 cm in thickness), while for the 0.20–1.60 m soil layer, the strategy was similar to strategies 1 and 2. Strategy 4 used 10 new theoretical locations of measurement every day. Precise water content estimates for thin layers were assumed to be available in the 0.00–0.20 m soil layer as in strategy 2. The layers for water content estimates in the 0.20–1.60 m were similar to those of strategies 1, 2, and 3. Results showed that the spatial variability of the daily actual evapotranspiration may not be negligible, and differences from approximately ±1.0 mm d ^–1 to ±3.0 mm d ^–1 were calculated between the four sampling strategies. Strategy 1 gave the worst results, because variations in the water content of the top soil layers were neglected, and thus the daily evapotranspiration was underestimated. Strategy 2 led to a considerable variability for estimating daily evapotranspiration which was explained by the effect of the spatial variability due to the daily site sampling for the top soil layers (0 to 0.2 m). Strategy 3 appeared to be the best practical compromise between practical field considerations and the necessity to obtain accurate evapotranspiration measurements. The accuracy of daily evapotranspiration could reach ± 0.5 mm d^–1, and could be further improved by increasing the number of measurement sites. The best results were obtained with strategy 4, although such a destructive and time-consuming strategy is not likely to be practical.     

Irrigation scheduling and water availability at watercourse command

A model that simulated the irrigation schedules of a farm at watercourse command was developed to predict net farm return, benefit-cost ratio, water use, percent water utilized, deep percolation, rainfall contribution and net return per unit of water applied including rainfall. Schedules for three selected farms on a watercourse command of Tw #62394L from MONA, Sargodha, Pakistan were simulated with 3 fixed-rotation and 2 demand strategies to evaluate the allowable soil water depletion criteria. Evaluation of the simulations (1973–82) showed that the water availability reduced the net farm return of 15 and 31% at the middle and tail farms, respectively, from that of the head farm. Therefore, the existing water allocation procedure (WARABANDI) should include watercourse conveyance losses to provide equitable water distribution on a watercourse command. Demand water availability can increase the net farm return of 25 and 26% in strategies 4 and 5, respectively, by changing the fixed-rotation system to a demand system. Changing the fixed-rotation system to a demand system requires either the use of existing private tubewells or the installation of new private tubewells.     

A comparison between the gross energy requirements of different irrigation systems in Israel

Summary The energy requirements for manufacturing irrigation equipment were evaluated from a survey of a number of factories and workshops in Israel.Based on the results obtained and the life span of the components, the annual amortization of energy by high-pressure (overhead sprinklers), medium-pressure (undertree sprinklers and sprayers) and low-pressure (drip lines) irrigation systems was calculated for citrus orchards and cotton crops as irrigated in Israel. For citrus orchards a low-pressure sprayer system amortized 1.5 GJ ha^–1 y^–1 more energy than a medium-pressure undertree sprinkler system, and 2.7 GJ ha^–1 y^–1 more than a high-pressure, overhead sprinkler system. For irrigating a cotton crop, the low-pressure drip system used 6.8 GJ ha^–1 y^–1 more embodied energy than the movable, high-pressure overhead sprinkler system.The annual energy invested in irrigation water conveyance through the National Water Carrier, at the current hydraulic pressure of 500 kPa at the farm gate, varies for a cotton crop from 20 to 45 GJ ha^–1 y^–1 in the northern region and from 70 to 215 GJ ha^–1 y^–1 in the southern region of Israel, when irrigated with 4,050 m³ ha^–1. For a citrus orchard this energy input varies from 60 to 75 GJ ha^–1 y^–1 in the central region and from 120 to 375 GJ ha^–1 y^–1 in the southern regions, when irrigated with 7,200 m³ ha^–1. For obtaining the same yield in the south as in the north, the energy input for water conveyance has to be increased by 12% in the case of a cotton crop and by 7% in the case of a citrus orchard. Thus, in the north the annual energy amortization of a dripline irrigation system amounts to one third of that expended on water conveyance but in the south amounts to one-eighteenth or less, indicating the large regional dependency of energy inputs for irrigation.Calculations show that the reduction in energy requirement for water conveyance needed by irrigation systems operating at lower pressures compensates for their higher energy losses in system amortization. For example, in citrus irrigation the substitution of medium-pressure undertree sprinkler systems for high-pressure overhead sprinkler systems was calculated to save 8% of the total energy expenditure for water conveyance to the farm gate. This would amount to a saving of 7 GJ ha^–1 y^–1 for citrus in the central region and of 8 GJ ha^–1 y^–1 in the south. For cotton the substitution of low pressure dripline systems for high-pressure overhead sprinkler systems could save 16% of the total energy expenditure for pressurized water conveyance. This would amount to a saving of 8 GJ ha^–1 y^–1 in the northern region increasing to 10 GJ ha^–1 y^–1 in the south, taking into account a higher irrigation water requirement.Contribution from the Agricultural Research Organization, Bet Dagan, Israel. No. 1589-E, 1985 series     

Farm types for beef production and their economic success in a mountainous province of northern Vietnam

The objective of this study was to compare the management and economic success of beef production by three types of farm in northwestern Vietnam. The potential of household farms to supply beef for the market and their competition with large farms were examined.The fieldwork was done in 2007 on 73 farms consisting of 58 small mixed farms (small farms), 10 medium mixed farms (medium farms) and 5 specialised large-scale beef farms (large farms) in Son La province. The three types of farm differed in ethnicity (Thai, H’mong, and Kinh), remoteness (lowland, highland), production objectives (subsistence, market output), degree of specialization (mixed farm, specialised beef farm) and integration of production (single farmers, cooperative). Data on biological productivity, inputs and outputs, and the social contribution of cattle production were collected by household and key person interviews, participatory rural appraisal tools and cattle body measurements. Economic values were derived by assessment of market or replacement costs. Quantitative data analysis was done with linear models (PROC GLM) in the SAS software (version 9.1).Lowland small farms had higher costs for cattle production than the highland farms (0.8 Mill. VND head⁻¹ year⁻¹ compared with 0.02 Mill. VND head⁻¹ year⁻¹, respectively). The large farms had high production costs, with an average of 2.5-3.6 Mill. VND head⁻¹ year⁻¹. Cattle brought high benefits of non-cash values to the household farms. The total revenue from cattle was in the range 4.5-11.5 Mill. VND head⁻¹ year⁻¹, which depended on the use of non-market functions of cattle on the household farm. The value of net benefit/kg live weight (LW) of lowland small farms with an average of 39,000 VND/kg LW was significantly higher than that of the medium and small farms in the highlands (26,000 VND/kg LW). However, the small farms kept fewer cattle than the medium farms (average of 2-4 cattle/farm compared with 9 cattle/farm, respectively) because of forage and labour shortages and have no option to further develop cattle production. Keeping larger numbers of cattle based on available natural pasture brought high benefit from stock value as farm liquidity to only the medium farms. This was the most promising type of farm for future development of beef production, given its actual success and the availability of underutilised resources. Large-scale farms suffered high economic losses of 0.3-1.4 Mill. VND cattle⁻¹ year⁻¹, due to the lack of professional management, high feed costs and low animal performance, and showed no potential for developing cattle production.     

Controlling nitrogen release from farm ponds with a subsurface outflow device: Implications for improved water quality in receiving streams

The retention of nutrients in farm ponds has many potential benefits, including reduction of nitrogen and phosphorus (promoters of eutrophication) in receiving streams. The aim of this study was to evaluate the efficacy of a commercial subsurface pond outflow control device (Pond Management System™) on nutrient retention in farm ponds. Four ponds of similar size and water chemistry in the upper Tar River basin of North Carolina, USA were studied; three were equipped with the pond outflow control device and one was retained without a device (normal surface outflow) that served as a reference site. Water samples were collected monthly from each pond at 0.3 m intervals from the surface to 2.1 m at a fixed station adjacent to the pond standpipe and from the pond outflow pipe from March to October 2005. The water samples were analyzed for total Kjeldahl nitrogen (N), total phosphorus (P), chlorophyll a, and a suite of other physicochemical variables. In ponds with the subsurface outflow device, the mean N concentrations in the outflow were substantially less (6.2–20.7%) than concentrations at the pond surface. Concentrations of N in the outflow were similar to N concentrations at intermediate pond depths (0.9–1.5 m), the depth of the outflow devices, indicating water was drawn from these depths and that N was being retained in the surface layers of the pond. Also, mean water temperatures were 1.1–1.9 °C cooler at intermediate depths compared to the surface, suggesting potential application of the outflow device for minimizing warm water outflows to receiving streams. These results provide evidence that under these conditions a subsurface pond outflow device can reduce nutrient release to receiving streams, thereby increasing overall stream water quality.     

Irrigation for crops in a sub-humid environment

Summary In this paper the soil water balance model developed and tested in Part III (Mason and Smith, 1980) for soybeans grown in the variable rainfall environment of the Namoi Valley of New South Wales was used to investigate the potential advantages of a computer-based system of irrigation scheduling. The advantages were evaluated using historical rainfall data for the 25 seasons from 1953/54 to 1977/78. The effects on irrigation efficiency of soil water holding capacity, the allowable soil water deficit prior to irrigation, and ordering irrigation water in advance were evaluated with the model. Reducing the allowed deficit prior to irrigation by 20% compared to the recommended level increased the number of irrigations by an average of 2.8 per year and irrigation requirements by 0.73 X 10³ m³ ha^–1. The need to order water 6 days in advance because of delays in delivery also increased requirements by 1.46 X 10³ m³ ha^–1 due to a reduced ability to utilize natural rainfall. Average farm irrigation efficiencies calculated from actual pumping records were found to be low by world standards for the 3-year period 1975/76 to 1977/78. It was concluded that if increased production per unit of water became a high priority in the Namoi Valley, then irrigation efficiency for the three year period discussed could have been increased from 35 to 47%, a saving of 1.3 X 10³ m³ ha^–1 year^–1.     

Effects of water table management on soil salinity and alfalfa yield in a semi-arid climate

A lysimeter experiment was conducted to investigate the effect of water table management (WTM) on distribution of soil salinity and annual alfalfa (Medicago scutellata) yield. Subirrigations with three levels of water table namely, 0.5 (WT0.5), 0.7 (WT0.7), and 1.0 m (WT1.0) and a free drainage (FD) conventional irrigation treatment were selected for this study. All treatments were arranged in a complete randomized block design with three replicates. The results of this study indicated that the average soil electrical conductivity of the saturated extract (ECe) in the root zone gradually increased and exceeded the designated crop threshold value (4 dS/m) after the first forage harvest in subirrigated lysimeters. A higher salt accumulation was observed at the WT0.5 treatment. The average dry matter yield of annual alfalfa in WT0.5 and WT0.7 treatments was found to be 52 and 73% higher compared with the control treatment, respectively.     

Raising surface water levels in peat areas with dairy farming: Upscaling hydrological, agronomical and economic effects from farm-scale to local scale

Raising surface water levels in peat areas is a measure to reduce soil subsidence, to prevent decay of wooden foundations and to stimulate wet nature restoration and reduce greenhouse gas emissions. However, in these areas dairy farms are present and farming at wetter soils is difficult due to lower bearing capacity of the soil for cattle and machines. Water boards are responsible for the water management of peat areas and thus have to evaluate the effects of water management strategies for the different land use functions. Therefore the hydrological, agronomical and economic effects of different surface water levels are calculated for dairy farms. The ‘Waterpas’ model is used to simulate hydrological effects, dairy farm management and economic results for different meteorological years. The raised surface water level causes a decrease in gross grass yield and a reduction in grass quality. This leads to higher costs and less farmers’ income relative to a reference situation with a freeboard of 60 cm. Raising the surface water increases the average costs for farmers with €89 ha⁻¹ year⁻¹ for a freeboard of 50 cm, €170 ha⁻¹ year⁻¹ for a freeboard of 40 cm and €239 ha⁻¹ year⁻¹ for a freeboard of 30 cm.However, water boards are not only interested in the effects for individual farms, but also for an entire region. A new spatial method was developed for upscaling from farm to polder level. For grassland fields in a typical Dutch peat area classes can be distinguished using GIS data on soil type, soil surface elevation, surface water levels, locations of farms and farm characteristics. The classification is based on 4 classes of freeboards of the grassland fields and 7 typical distributions of grassland fields within a dairy farm. The farm economics were simulated for these typical classes. An increase in costs was simulated for the whole polder Zegveld (1400 ha grassland) of €119,000 year⁻¹ at 10 cm surface water level rise; €133,000 year⁻¹ at 20 cm surface water level rise and €185,000 year⁻¹ at 30 cm surface water level rise.For an integral environmental evaluation of changing hydrological conditions it is advised to incorporate effects on nutrient emission to groundwater and surface water and emission of ammonia and greenhouse gases to the atmosphere.     

Irrigation for crops in a sub-humid environment VII. Evaluation of irrigation strategies for cotton

Summary Empirical functions to predict the nitrogen uptake, increase in LAI and minimum leaf water potential (LWP) of cotton were incorporated into a water balance model for the Namoi Valley, N.S.W. A function was then developed to describe the lint yield of irrigated cotton as a function of water stress days at 4 stages of development, total nitrogen uptake and days of waterlogging. A water stress day was defined as predicted minimum leaf water potential less than -1.8 MPa up to 90 days after sowing and -2.4 MPa there-after; stress reduced yield by up to 40 kg lint ha^–1 d^–1 with greatest sensitivity at 81–140 days after sowing and when N uptake was highest. Nitrogen uptake was reduced by 0.98 kg per ha and yield reduced by 33.2 kg lint ha^–1 for each day of waterlogging. The model was used to evaluate various irrigation strategies by simulating production of cotton from historical rainfall data. With a water supply from off farm storage, net returns ($ M1^–1) were maximized by allocating 7 Ml ha^–1 of crop. The optimum practice was not to irrigate until 60 days from sowing and until the deficit in the root zone reached 50%. When the supply of water was less than 7 Ml ha^–1 there was no advantage in either delaying the start of irrigation or irrigating at a greater deficit; it was economically more rational to reduce the area shown or, if already sown, to irrigate part with 6 Ml ha^–1 and leave the rest as a raingrown crop. Irrigation decisions are compromises between reducing the risk of water stress and increasing the risk of waterlogging. The simulation showed that there is no single set of practices that is always best in every season; in a number of seasons practices other than those which on average are best, give better results.     

Reductions in water use following rehabilitation of a flood-irrigated area on the Murray River in South Australia

Water use was monitored during rehabilitation of flood-irrigated farms in the lower Murray River of South Australia. Ten irrigation offtakes at six farms were monitored over a period of two years during the rehabilitation process. Full rehabilitation consisted of improved inlet structures, flow metering, elimination of water leaks, laser levelling of paddocks, and construction of re-use systems to recycle excess surface irrigation runoff. Partial rehabilitation consisted of the same improvements with the exception of the re-use system. The mean water use per watering of 0.61 ± 0.08 Ml/ha for the fully rehabilitated farm was approximately one third of that for non-rehabilitated farms (1.89 ± 0.15 Ml/ha) and two thirds of that for partially rehabilitated farms (0.99 ± 0.07 Ml/ha). These differences were statistically significant at the 95% confidence level. A large improvement in efficiency of water use was achieved by upgraded water delivery infrastructure and laser levelling of paddocks. Considerable improvement in water use efficiency was also gained, however, only by installation of re-use systems. It is expected that the overall rehabilitation of irrigation infrastructure will result in a significant reduction of water extracted from the river for flood irrigation in this region. Further longer term monitoring is required to confirm this.     

Pipe drainage in the Eastern Padano-Veneta plain in north-east Italy

A survey has been carried out in three Regions of north-eastItaly, Emilia Romagna, Veneto and Friuli Venezia Giulia, withfour objectives: (a) to update the statistics on the use of pipedrainage; (b) to describe the farms that adopt this technique asa replacement of the traditional surface drainage methods;(c) to characterise the features of the drained soils and of thedrainage systems, and (d) to assess their performance. Withalmost 70.000 ha drained, the Regions present almost all of thepipe drained area in Italy. Pipe drainage first saw applicationsin the last seventies and became increasingly popular during theeighties. Most pipe drainage systems are installed in heavysoils, where the underground waterlogging is due to seasonalperched water table or to infiltrations from rivers or sea. Due tothe pedoclimatic variability of the area, slightly differentsolutions in terms of design variables and installation criteriaare found, but the following features are quite common: drainspacing 11–13 m slope 0.1–0.3%, depth 0.9–1 m. The pipe drains discharge in open collectors (single systems). Anincreasing interest for the use of the drainage systems forsubirrigation purposes has been observed, particularly inVeneto, requiring some adjustment in the drainage design. Theperformance of drainage in terms of control of water tabledepth and maintenance requirements are considered satisfactoryin almost all the cases. The most effective solutions forsuccessfully managing the subirrigation are also described.The introduction of pipe drains allows to enlarge fields (to morethan 10 has) and to save time for carrying out the operations forcrop cultivation. The crop yield in pipe drained fields is slightly higherto that obtained in the traditionally drained fields, withthe exception of winter wheat.     

Compaction of sandy soils for irrigation management

Summary Several irrigation management experiments were conducted at different locations on sandy soils in Haryana State to overcome excessive permeability, poor soil moisture retention and storage in the root zone. Subsurface compaction to 30–40 cm depth created by 6 passes of a 1,500 kg tractor-driven iron roller, 24 to 48 h after irrigation, was found to be beneficial in reducing irrigation requirement. In general, yield of different crops was not affected significantly by surface rolling, except that of mustard which increased significantly. Slight increase in subsurface compaction, about 0.1 g cm^–3, increased the soil moisture retention and reduced the infiltration rate and saturated hydraulic conductivity. Hydraulic conductivity was a better parameter than bulk density for evaluating the effect of rolling. The depth of irrigation water applied in rolling treatment was about 58–74% that of the no-rolling control. Compaction thus reduced water input to these sandy soils without adversely affecting the crop yield.     

Identifying and controlling critical sources of farm phosphorus imbalances for Vermont dairy farms

Lake Champlain, located between Vermont, New York, and Quebec exhibits eutrophication mainly due to continuing phosphorus (P) losses from upstream nonpoint source areas. Several state and local agencies have initiated efforts aimed at assessing and identifying critical sources areas for P loss. To augment these efforts, accounting of farm P inputs (in purchased animal feed and fertilizers) and P outputs (in milk, meat, or off-farm sales of harvested crops or other products) is needed as a means of determining potential P build-up in farm soils. When farm P inputs exceed P outputs, P surplus occurs on the farm. This leads to potential soil-P accumulations and risk of P loss in runoff, negatively impacting the quality of receiving water bodies. In this study, a combination of farm record data and a model-based approach, using the Integrated Farming System Model (IFSM), was used to estimate farm P inputs and outputs, identify root causes of farm P imbalances, and explore viable P balancing strategies. Three Vermont dairy farms with varying farm systems (grass-based organic farm, fully confined farm, and a mixed system farm with high-producing confined dairy cows and grazing heifers) were studied. These farms were found to have P surpluses ranging from 5.5 kg/ha to 18.7 kg/ha on annual basis. This study also identified critical causes of P imbalances for each farm and suggested farm specific alternative strategies needed to address the P imbalances. By balancing farm P inputs and outputs, potential accumulation of soil-P can be prevented. As a result, maximum benefits can be obtained from land treatment measures implemented to control off-field P loss without the additional concern of continuing P build-up that could reduce their effectiveness.     

The diversity of rural livelihoods and their influence on soil fertility in agricultural systems of East Africa - A typology of smallholder farms

Technological interventions to address the problem of poor productivity of smallholder agricultural systems must be designed to target socially diverse and spatially heterogeneous farms and farming systems. This paper proposes a categorisation of household diversity based on a functional typology of livelihood strategies, and analyses the influence of such diversity on current soil fertility status and spatial variability on a sample of 250 randomly selected farms from six districts of Kenya and Uganda. In spite of the agro-ecological and socio-economic diversity observed across the region (e.g. 4 months year⁻¹ of food self-sufficiency in Vihiga, Kenya vs. 10 in Tororo, Uganda) consistent patterns of variability were also observed. For example, all the households with less than 3 months year⁻¹ of food self-sufficiency had a land:labour ratio (LLR) < 1, and all those with LLR > 1 produced enough food to cover their diet for at least 5 months. Households with LLR < 1 were also those who generated more than 50% of their total income outside the farm. Dependence on off/non-farm income was one of the main factors associated with household diversity. Based on indicators of resource endowment and income strategies and using principal component analysis, farmers’ rankings and cluster analysis the 250 households surveyed were grouped into five farm types: (1) Farms that rely mainly on permanent off-farm employment (from 10 to 28% of the farmers interviewed, according to site); (2) larger, wealthier farms growing cash crops (8-20%); (3) medium resource endowment, food self-sufficient farms (20-38%); (4) medium to low resource endowment relying partly on non-farm activities (18-30%); and (5) poor households with family members employed locally as agricultural labourers by wealthier farmers (13-25%). Due to differential soil management over long periods of time, and to ample diversity in resource endowments (land, livestock, labour) and access to cash, the five farm types exhibited different soil carbon and nutrient stocks (e.g. Type 2 farms had average C, N, P and K stocks that were 2-3 times larger than for Types 4 or 5). In general, soil spatial variability was larger in farms (and sites) with poorer soils and smaller in farms owning livestock. The five farm types identified may be seen as domains to target technological innovations and/or development efforts.     

Calibration of thermocouple hygrometers

Summary The objective of the research reported was to improve the calibration procedure used for thermocouple hygrometers. If the wet bulb temperature is defined by the maximum point-of-inflection voltage of the psychrometer output, then a unique point with maximum sensitivity to water potential is obtained (Figs. 2, 3). A predictive model based on calibration data at a few temperatures is used to obtain the psychrometer calibration slope at any temperature (eq. 11 to 15). Use of this model indicates that psychrometers differ from each other (Tables 4, 5) and therefore must be individually calibrated. Dewpoint hygrometers are shown to be less temperature sensitive than thermocouple psychrometers (Fig. 4) and have the added advantage of a voltage sensitivity nearly twice that of psychrometers, –7.0 X 10^–3 V kPa^–1 compared to –3.7 X 10^–3 V kPa^–1 at 25 °C. However, the accuracy of thermocouple hygrometers is critically dependent on the correct setting of the dewpoint cooling coefficient, particularly at temperatures less than (about) 15 °C. At 15 °C, for example, the coefficient must be set within 1 % of the true value in order for the correct voltage to have an error of less than 4%.Work supported by grants from the University of Natal Research Fund and the Department of Agriculture and Fisheries, South Africa     

Modelling the fate of water and nitrogen in the mixed vegetable farming systems of northern Tasmania, Australia

A combination of high input management systems, high annual rainfall and deep, permeable soils in northern Tasmania create conditions that are conducive to high drainage and nitrogen losses below the root zone. An understanding of the extent and mechanism of such losses will enable farm managers and their consultants to identify and implement more sustainable management practices that minimise potential adverse financial and environmental consequences. Analysing the fate of water and nutrients in farming systems is complex and influenced by a wide range of factors including management, soil characteristics, seasonal climate variability and management history of the paddock/farm in question. This paper describes a novel farm system modelling approach based on the model APSIM, for analysing the fate of nitrogen and water in mixed vegetable-based farming enterprises. The study was based on seven case farms across the Panatana catchment in northern Tasmania. Substantial simulated drainage losses (>100 mm average seasonal loss) were apparent for all crop and rotation elements across all farms in response to the surplus between crop water supply and crop water use. Crop nitrogen demand was found to be close to crop nitrogen supply for all crop and pasture rotation elements with the exception of potato, which had an average surplus nitrogen supply of 89 kg N/ha. This resulted in potato having much higher nitrate nitrogen leaching losses (32 kg N/ha) compared to other crops (<10 kg N/ha). Simulations suggest that practicable management options such as deficit-based irrigation and reduced N fertiliser rates will maintain current levels of productivity while reducing potential offsite N loss and generating significant financial savings via reduced input costs.