IMAGE: a physically-based one dimensional irrigation management model for soil salinity control

Abstract. This paper describes a study of 37 farms in the Batinah region of Oman where fodder crops and date palms are grown using saline irrigation water. Soil water salinities (ε_s) range from 2 to 50 dS m^–1. Soil water salinity depends on irrigation water quality and management factors such as the amount and frequency of irrigation and the area of the irrigation basin relative to the vegetation canopy. An irrigation management model for soil salinity control IMAGE has been developed, based on the salt balance of the profile assuming that the ε_s is in equilibrium with the irrigation water. The input parameters required to run the model include the annual water application, irrigation interval, soil textural class, potential evaporation, the ratio of crop canopy to irrigation basin area and the salinity of irrigation water. Verification of the model using rather uncertain data from a survey of the farms showed that this simple approach predicted ε_s to within 2.5 dS m^–1 in 82% of cases. The model showed that ε_s was highly sensitive to the size of irrigation basin and the amount and scheduling of irrigation, and so provides a tool for optimizing salinity management.     

Errors in the estimation of soil water properties and their propagation through a hydrological model

Abstract. The Agricultural Catchments Research Unit model (ACRU) includes a decision support system (DSS) for estimating the water content of soil at field capacity (θ _fc ) and wilting point (θ _wp ) when these characteristics are not directly measurable. Three methods of estimation are proposed: (a) based on silt and clay content and bulk density, (b) based on clay content only, and (c) based on soil series. These three pedotransfer functions are compared with respect to both the estimation of θ _fc and θ _wp and the propagation of errors when the actual evapotranspiration of a wheat crop (E) is predicted over the growing season by the ACRU model.
The standard error of estimation was between 0.066 and 0.082 m³/m³ for θ _fc , between 0.056 and 0.069 m³/m³ for θ _wp and between 29.9 and 34.8 mm of water for E. The method based on silt and clay contents and bulk density predicted θ _fc and θ _wp for non-swelling soils most precisely. The method based on soil series was better than other methods for swelling soils. It also performed better for estimating available water capacity and consequently for predicting E from a conceptual soil water model. The propagated error of estimating θ _fc and θ _wp using the DSS reached 15–18% of the simulated E. The error in the prediction of E can reach 26–30% when spatial variation in soil properties is also estimated.     

Heat pipe phenomenon in soil under reduced air pressure

We measured the heat flux, temperature distribution and water content of an unsaturated Ando soil under a constant temperature gradient and reduced air pressure to investigate the mechanism of latent heat transfer in the soil and its relationship to the distribution and circulation of soil water. As the air pressure decreased, the heat flux increased for the soil samples with an initial volumetric water content ( θ _ini) greater than 0.30 m³ m⁻³, but did not change for θ _ini less than 0.20. While the temperature gradient of the sample did not change for θ _ini greater than 0.30 m³ m⁻³, it did increase on the hotter side of the sample and decreased on the colder side for θ _ini less than 0.20. The water content did not change, and a homogeneous distribution of water content was observed for θ _ini greater than 0.30 m³ m⁻³. For θ _ini less than 0.20, the water content decreased on the hotter side and increased on the colder side, forming a large water content gradient. The large transfer of latent heat was caused by the circulation of water vapour and liquid water, which resulted in the homogeneous water distribution. We concluded that the soil functions as a heat pipe through a series of micro-heat pipes centred on the soil pores. Our experimental results will help to explain the transfer mechanism of latent heat in soil as a heat pipe phenomenon.     

Amelioration of saline–sodic soil with mildly saline water in the Songnen Plain, northeast China

Abstract. The saline–sodic soils of the dryland Songnen Plain in northeast China are only slowly permeable to fresh water because of their large content of montmorillinite clay and sodium bicarbonate. Use of slightly saline groundwater containing adequate dissolved calcium and magnesium for leaching and reclamation can potentially prevent dispersion of the clay soil particles during treatment. Amelioration was evaluated using shallow, mildly saline groundwater to irrigate sorghum–corn rotations in a two-year field experiment. After two growing seasons during which a total of 400 mm of leaching water was applied, in addition to some supplemental irrigation water, the average electrical conductivity (EC_e) of the top 1.2 m of the soil profile decreased from 14.5±3.5 to 2.7±0.2 dS m⁻¹, and the sodium absorption ratio (SAR_e) decreased from 35.3±4.1 to 10.1±2.5 (meq L⁻¹)^0.5. The soil physical properties were improved: infiltration rate with mildly saline groundwater increased from 12.1 to 42 mm h⁻¹. Salinity changes in the top 1.2 m of soil layers after 700 mm of leaching produced no further improvement. Crop yields produced on plots undergoing amelioration increased by 64–562% compared with the rainfed control. The improved soil conditions after leaching resulted in 59–548% greater crop yields.     

Determination of soil salinity based on WET measurements using the concept of salinity index

We evaluated the Malicki and Walczak model (MW) and its appropriately modification (MMW) in the prediction of the electrical conductivity of the soil solution (σ_p), utilizing the WET dielectric sensor. In the MMW approach, the prediction of σ_p requires determination of the WET‐based salinity index (X_s) and clay and sand contents of the soil. MMW appears to be more effective than MW in all cases except for the cases of finer soils when σ_p > 3 dS m^?1.     

Investigating preferential flow in a large intact soil block under pasture

Abstract. A large soil block was constructed to determine the importance of preferential flow routes compared with matric flow pathways at a pasture site in mid-Devon. The sandy loam soil was well structured and uniform. The soil block measured 5 m×3 m×1 m and was instrumented with an array of 54 tensiometers, TDR wave guides and suction samplers connected to an in situ chloride analysis system. Four steady state irrigation experiments were conducted with a range of rainfall intensities. During each experiment chloride and nitrate tracers were applied and the patterns of movement were observed. Although the application of tracer was uniform and the soil was relatively homogeneous, there was large variability across the block in terms of time taken to reach the peak concentration (TPC) and the peak concentration itself. About 44 samplers operated at the greatest intensities (10–2 mm h⁻¹) and only 35 at the smallest (1 mm h⁻¹). No relationship was found between TPC and depth. The fastest TPC and largest concentrations were associated with the greatest rainfall intensities. Relative importance of the individual water pathways was a function of soil heterogeneity: parts of the soil block were highly active with several pathways having short TPCs and conductivities in excess of 4 m day⁻¹ whereas other areas had longer TPCs and conductivities of 1–2 m day⁻¹. The pattern was also dynamic, with conductivities of the pathways changing through time, though most of the faster pathways maintained their greater conductivities for more than one year.     

Mapping soil salinity in 3‐dimensions using an EM38 and EM4Soil inversion modelling at the reconnaissance scale in central Morocco

Large areas of Morocco require irrigation and although good quality water is available in dams, farmers augment river water with poorer quality ground water, resulting in salt build‐up without a sufficient leaching fraction. Implementation of management plans requires baseline reconnaissance maps of salinity. We developed a method to map the distribution of salinity profiles by establishing a linear regression (LR) between calculated true electrical conductivity (σ, mS/m) and electrical conductivity of the saturated soil‐paste extract (ECe, dS/m). Estimates of σ were obtained by inverting the apparent electrical conductivity (ECa, mS/m) collected from a 500‐m grid survey using an EM38. Spherical variograms were developed to interpolate ECa data onto a 100 m grid using residual maximum likelihood. Inversion was carried out on kriged ECa data using a quasi‐3d model (EM4Soil software), selecting the cumulative function (CF) forward modelling and S2 inversion algorithm with a damping factor of 3.0. Using a ‘leave‐one‐out cross‐validation' (LOOCV), of one in 12 of the calibration sites, the use of the q‐3d model yielded a high accuracy (RMSE = 0.42 dS/m), small bias (ME = ?0.02 dS/m) and Lin's concordance (0.91). Slightly worse results were obtained using individual LR established at each depth increment overall (i.e. RMSE = 0.45 dS/m; ME = 0.00 dS/m; Lin's = 0.89) with the raw EM38 ECa. Inversion required a single LR (ECe = 0.679 + 0.041 × σ), enabling efficiencies in estimating ECe at any depth across the irrigation district. Final maps of ECe, along with information on water used for irrigation (ECw) and the characterization of properties of the two main soil types, enabled better understanding of causes of secondary soil salinity. The approach can be applied to problematic saline areas with saline water tables.     

Strategy for long term use of saline drainage water for irrigation in semi-arid regions

In arid and semi-arid regions, effluent from subsurface drainage is often saline and in the absence of a natural outlet, its disposal is a serious environmental threat. A field experiment was conducted for 7 years using drainage water of different salinity levels (EC_iw=6, 9, 12 and 18.8 dS/m) for irrigation of wheat during the dry winter season. The objective was to find whether crop production would still be feasible and soil salinity would not be increased unacceptably by this practice. The experimental crop was wheat during the winter season and pearl-millet and sorghum in the rainy season, grown on a sandy loam soil provided with subsurface drainage system. All crops were given a pre-plant irrigation with non-saline canal water and subsequently, saline drainage water of different salinity levels was used for the irrigation of wheat as per the treatment. On an average, the mean yield reduction in wheat yield at different EC_iw was 4.2% at 6, 9.7% at 9, 16.3% at 12 and 22.2% at 18.8 dS/m. Pearl-millet and sorghum yields decreased significantly only where 12 dS/m or higher salinity water was applied to previous wheat crop. The high salinity and sodicity of the drainage water increased the soil salinity and sodicity in the soil profile during the winter season, but these hazards were eliminated by the subsurface drainage during the ensuing monsoon periods. The results obtained provide a promising option for the use of poor quality drainage water for the irrigation of winter wheat without undue yield reduction and soil degradation.     

Pollution of carbonate soils in a Mediterranean climate due to a tailings spill

The retention walls in a pond containing the residues from the pyrite mine of Aznalcóllar (southern Spain) broke open on 25 April 1998, spilling approximately 6 × 10⁶ m³ of polluted water and toxic tailings, which affected some 55 km². Drying and aeration of the tailings resulted in oxidation, forming an acidic solution with high pollutant contents, the effects of which were studied in a calcareous soil. The infiltration of this solution markedly affected only the first 12 mm of the soil, where strong acidification caused the weathering of the carbonates, and where the fine mineral particles were hydrolysed. The SO₄²⁻ ions in the acidic solution precipitated almost entirely at this depth, forming gypsum, hydroxysulphates and complex sulphates. The Fe³⁺ ions also precipitated there, mainly in amorphous or poorly crystallized forms, adsorbing to As, Sb, Tl and Pb dissolved in the acidic solution. The Al³⁺ ions, though partly precipitating in the acidic layer, accumulated mostly where the soil pH exceeded 5.5 (12–14 mm in depth). They did so primarily as amorphous or poorly crystallized forms, adsorbing to Cu dissolved in the acidic solution. The Zn²⁺ and Cd²⁺ ions accumulated mainly at pH > 7.0 (19–21 mm in depth), being adsorbed chiefly by clay mineral. After 15 months, only the first 20 mm of the soil were acidified by the oxidation of the tailings and most of the pollutants did not penetrate deeper than 100 mm. Consequently, the speed of the cleanup of the toxic spill is not as important as a thorough removal of tailings together with the upper 10 cm of the soil.     

不同类型咸水滴灌对土壤盐分的影响及棉花产量响应

棉花是鲁北平原种植的重要经济作物,合理利用微咸水和咸水资源是解决棉花季节干旱问题的重要途径。通过田间小区试验,以淡水滴灌处理为对照,设置不同盐分梯度的咸水滴灌处理,研究2种类型咸水滴灌对棉田土壤水分和盐分的分布影响以及棉花产量的响应。结果表明,咸水滴灌条件下主要影响棉田40~100 cm土壤水分的变化,碳酸氢钠型和氯化钠型咸水处理对土壤含水量的影响没有显著差异。利用EC值低于8 d S·m~(-1)的咸水进行补灌,棉田0~40 cm土壤盐分积累不明显,灌溉水EC值为10 d S·m~(-1)的氯化钠型咸水灌溉在0~100 cm土壤盐分有明显的积累。滴灌补灌EC值不大于6 d S·m~(-1)的碳酸氢钠型咸水和不大于8 d S·m~(-1)的氯化钠型咸水对棉花产量没有明显的影响,滴灌补灌7 d S·m~(-1)碳酸氢钠型和10 d S·m~(-1)氯化钠型咸水明显降低棉花产量。从土壤盐分的积累和棉花产量来看,在鲁北平原可以利用6 d S·m~(-1)咸水滴灌对棉花进行补灌;利用咸水滴灌,要同时考虑灌溉水盐分的数量和盐分组成,碳酸氢钠型咸水要更加谨慎利用。     

Pot study on wheat growth in saline and waterlogged compacted soil: II. Root growth and leaf ionic relations

Soil compaction affects plant growth by causing increased resistance to root penetration and a decreased uptake of water and ions. A pot experiment was conducted to study the effect of soil compaction in conjunction with the soil salinity and waterlogging on root growth and leaf ionic composition of two wheat genotypes (Aqaab and MH-97). Compaction was achieved at a 10% soil moisture content by dropping 5 kg weight, controlled by a tripod stand for 20 times from 0.6 m height on a wooden block placed inside the soil-filled pots. Soil bulk density of non-compact and compact treatments was measured as 1.21 and 1.65 Mg m⁻³, respectively. The desired salinity level (15 dS m⁻¹) was developed by mixing required amount of NaCl in the soil before filling the pots. Waterlogging was developed by flooding the pots for 21 days both at tillering and booting stages. Compaction significantly reduced the root length density (RLD) of both the wheat genotypes while the combined effect of compaction×salinity was more drastic on root length density than compaction alone. Waterlogging however, did not decrease the root length density, rather it mitigated the effect of compaction. Compaction decreased the concentration of K⁺ and K⁺:Na⁺ ratio in leaves. Salinity also decreased the concentration of K⁺ and K⁺:Na⁺ ratio, but increased the concentrations of Na⁺ and Cl⁻ in the crop leaves. Salinity and compaction interacted to cause a greater reduction in K⁺ concentration and the K⁺:Na⁺ ratio, while there was lesser increase in the concentrations of Na⁺ and Cl⁻ compared with salinity alone. Waterlogging also decreased the concentration of K⁺ and K⁺:Na⁺ ratio in leaves. It intensified the effect of salinity but decreased the effect of compaction on leaf ionic composition. Therefore, the effect of compaction on root growth and ion uptake is more severe under salt-affected soil conditions than under normal soil conditions while occasional waterlogging of a compact soil for a few days makes the soil conditions favorable for root growth both under non-saline as well as saline soil conditions. Also, the performance of a genotype in stressed environment is related to maintenance of higher root length density, leaf K⁺ concentration and K⁺:Na⁺ ratio and lower leaf Na⁺ and Cl⁻ concentrations.     

基于T-S模糊模型的TDR土壤水分传感器标定方法研究

利用TDR土壤水分传感器实现土壤水分数值的精确采集具有现实意义[1-3]。为了获得TDR土壤水分传感器水分含量测量所对应输出电压与实际土壤含水量的良好对应关系,在采用取土实验测量法获得实际水分含量数值与对应输出电压数值的基础上,利用Takagi-Sugeno模型逼进精度高、后件参数线性化等优点[2,4]对TDR土壤水分传感器     

滨海重度盐碱地微咸水滴灌水盐调控及月季根系生长响应研究

滨海盐碱地是滨海地区重要的土地资源,随着滨海地区城镇化进程及生态文明建设的发展,迫切需要低成本、快速、可持续的滨海盐碱地原土植被构建技术。针对滨海盐碱地原土建植与咸水/微咸水资源的利用,该研究以月季(Rosa chinensis)为例,采用微咸水滴灌技术进行滨海盐碱地水盐调控植被构建。试验在渤海湾曹妃甸区吹沙造田形成的典型沙质滨海盐渍土上进行,设计了灌溉水电导率(ECiw)为0.8、3.1、4.7、6.3、7.8 dS/m的5个处理,研究滴灌水盐调控对土壤盐分淋洗及月季根系生长和分布特征的影响。结果表明:在渤海湾滨海地区气候条件下,先进行淡水滴灌盐分强化淋洗和缓苗灌溉,随后采用7.8 dS/m的微咸水滴灌,0～100 cm土层土壤盐分得到了有效的淋洗,尤其是根层0～40 cm土壤盐分经过一个月左右,由初始28.33 dS/m降低到均小于4 dS/m,一个低盐适生的土壤环境得到快速营造;随着ECiw的增加,0～40 cm土层土壤最终趋于稳定的盐分呈增加趋势,土壤脱盐过程可以被logistic方程描述,脱盐过程可划分为快速脱盐、缓慢脱盐和盐分趋于稳定3个阶段;94%以上的月季根系主要分布在0～20cm的表层土壤中,随着ECiw的增加,根系生物量显著降低,根系受盐分胁迫生理干旱影响向土壤深处生长以扩大水分空间。研究认为,采用短期淡水滴灌盐分强化淋洗和缓苗淡水滴灌、随后进行微咸水滴灌的方法,可以实现土壤盐分的快速淋洗并维持在较低水平,但受盐分对根系生长的影响会作用于植物地上部分生长及植物存活,因此需要结合植物耐盐性及生产目标(产量、景观)确定适宜灌溉水矿化度阈值。     

Water quality implications of dairy slurry applied to cut pastures in the northeast USA

Abstract. Nitrate nitrogen (NO₃-N) leaching from animal production systems in the northeast USA is a major non-point source of pollution in the Chesapeake Bay. We conducted a study to measure NO₃-N leaching from dairy slurry applied to orchardgrass ( Dactylis glomerata L., cv. Pennlate) using large drainage lysimeters to measure the direct impact of four rates of slurry (urine and faeces) N application (0, 168, 336, 672 kg N ha⁻¹ yr⁻¹) on NO₃-N leaching on three soil types. We then used experimentally-based relationships developed earlier between stocking density and NO₃-N leaching loss and leachate NO₃-N concentration to estimate the added impact of animal grazing. Nitrate N leaching losses from only dairy slurry applied at the 0, 158, 336, and 672 kg N ha⁻¹ yr⁻¹ rates were 5.85, 8.26, 8.83, and 12.1 kg N ha⁻¹ yr⁻¹, respectively with corresponding NO₃-N concentrations of 1.60, 2.30, 2.46, and 3.48 mg l⁻¹. These NO₃-N concentrations met the 10 mg l⁻¹ US EPA drinking water standard. However, when a scenario was constructed to include the effect of NO₃-N leaching caused by animal grazing, the NO₃-N drinking water standard was calculated to be exceeded.     

Revised estimates of the carbon mitigation potential of UK agricultural land

Abstract. The soil sequestration components of recent estimates of the carbon mitigation potential of UK agricultural land were calculated on the basis of a percentage change to the soil carbon stock present in the soil. Recent data suggest that the carbon stock of soil in UK arable land has been overestimated, meaning that potential soil carbon sequestration rates were also overestimated. Here, we present a new estimate of the carbon stock in UK arable land, and present revised estimates for the carbon mitigation potential of UK agricultural land. The stock of soil organic carbon in UK arable land (0–30 cm) is estimated to be 562 Tg, about half of the previous estimate. Consequently, the soil carbon sequestration component of each mitigation option is reduced by about half of previously published values. Since above-ground carbon accumulation and fossil fuel carbon savings remain unchanged by these new soil carbon data, options with a significant non-soil carbon mitigation component are reduced by less than those resulting from soil carbon sequestration alone. The best single mitigation option (bioenergy crop production on surplus arable land) accounts for 3.5 Tg C yr⁻¹, (2.2% of the UK's 1990 CO₂-carbon emissions), whilst an optimal combined land-use mitigation option accounts for 6.1 Tg C yr⁻¹ (3.9% of the UK's 1990 CO₂-carbon emissions). These revised figures suggest that through manipulation of arable land, the UK could, at best, meet 49% of its contribution to the EU's overall Kyoto CO₂-carbon emission reduction target (8% of 1990 emissions), and 31% of the greater target accepted by the UK (12.5%). Even these reduced estimates show a significant carbon mitigation potential for UK arable land.     

滴灌模式对棉花根系分布和水分利用效率的影响

理解膜下滴灌参数对土壤盐分运移和作物生长的影响是制定科学滴灌制度、合理利用水资源的重要环节。毛管布置方式和滴灌水质是膜下滴灌的重要参数,为研究其对土壤盐分变化、棉花根系分布及水分利用效率的影响,设计了2种毛管布置方式（一管四行（Ms）和一管两行（Md））和3个滴灌水质水平（淡水0.24?dS/m、微咸水4.68?dS/m、咸水7.42?dS/m）。结果表明,滴管布置方式对土壤盐分变化和根系分布有显著影响。在相同滴灌水质条件下,Ms处理有利于降低棉花根区土壤含盐量。所有处理根系主要分布于0～40?cm土层内,矿质水滴灌时Md中根系受抑制程度明显高于Ms,但其主要影响根系密度δR＞0.5?kg/m3区域的分布范围,对δR＞0.2?kg/m3区域范围分布无明显影响。生育期内棉花总耗水量随滴灌水矿化度的上升而降低,与滴管布置无关。相对淡水滴灌而言,矿质水滴灌时Ms处理产量有所降低,但其水分利用效率随灌水矿化度上升而升高;而Md处理产量和水分利用效率均随灌水矿化度上升而下降。     

Net nitrogen mineralization and nitrification rates in forest soil in northeastern Taiwan

We used a laboratory incubation approach to measure rates of net N mineralization and nitrification in forest soils from Fu-shan Experimental Forest WS1 in northern Taiwan. Net mineralization rates in the O horizon ranged from 4.0 to 13.8 mg N kg⁻¹ day⁻¹, and net nitrification rates ranged from 2.2 to 11.6 mg N kg⁻¹ day⁻¹. For mineral (10–20 cm depth) soil, net mineralization ranged from 0.06 to 2.8 mg N kg⁻¹ day⁻¹ and net nitrification rates ranged from 0.02 to 2.8 mg N kg⁻¹ day⁻¹. We did not find any consistent differences in N mineralization or nitrification rates in soils from the upper and lower part of the watershed. We compared the rates of these processes in three soil horizons (to a soil depth of 30 cm) on a single sampling date and found a large decrease in both net N mineralization and nitrification with depth. We estimated that the soil total N pool was 6,909 kg N ha⁻¹. The present study demonstrates the importance of the stock of mineral soil N in WS1, mostly organic N, which can be transformed to inorganic N and potentially exported to surface and ground water from this watershed. Additional studies quantifying the rates of soil N cycling, particularly multi-site comparisons within Taiwan and the East Asia–Pacific region, will greatly improve our understanding of regional patterns in nitrogen cycling.     

Methane and nitrous oxide fluxes from a farmed Swedish Histosol

Fluxes of the greenhouse gases methane (CH₄) and nitrous oxide (N₂O) from histosolic soils (which account for approximately 10% of Swedish agricultural soils) supporting grassley and barley production in Sweden were measured over 3 years using static chambers. Emissions varied both over area and time. Methane was both produced and oxidized in the soil: fluxes were small, with an average emission of 0.12 g CH₄ m⁻² year⁻¹ at the grassley site and net uptake of −0.01 g CH₄ m⁻² year⁻¹ at the barley field. Methane emission was related to soil water, with more emission when wet. Nitrous oxide emissions varied, with peaks of emission after soil cultivation, ploughing and harrowing. On average, the grassley and barley field had emissions of 0.20 and 1.51 g N₂O m⁻² year⁻¹, respectively. We found no correlation between N₂O and soil factors, but the greatest N₂O emission was associated with the driest areas, with < 60% average water-filled pore space. We suggest that the best management option to mitigate emissions is to keep the soil moderately wet with permanent grass production, which restricts N₂O emissions whilst minimizing those of CH₄.     

Calibration equations for Diviner 2000 capacitance measurements of volumetric soil water content of six soils

Abstract. Calibration coefficients for a Diviner 2000 capacitance sensor were developed under laboratory conditions for soils of six textures. The calibration equations, derived by regression analysis, significantly ( P <0.001) related Diviner 2000 measurements of scaled frequency (SF) with volumetric soil water content of the soil. In all cases the calibration accounted for >93% of the variation ( R ² adjusted) with the volumetric water content of the soil.     

Monitoring of water erosion on arable farms in England and Wales. 1990–94

Abstract. The incidence of soil water erosion was monitored in 12 erosion-susceptible arable catchments ( c . 80 fields) in England and Wales between 1990 and 1994. Factors associated with the initiation of erosion were recorded, and the extent of rills and gullies measured. Approximately 80% of the erosion events were on land cropped to winter cereals. In 30% of cases, the initiation of erosion was linked to valley floor features, which concentrated runoff. Poor crop cover, wheelings and tramlines were also assessed as contributory factors in 22%, 19% and 14% of cases, respectively. In c . 95% of cases rainfall events causing erosion were ≥10 mm day⁻¹ and c . 80% were >15 mm day⁻¹. Erosion was also associated with maximum rainfall intensities of >4 mm h⁻¹ for c . 90% of cases and >10 mm h⁻¹ for c . 20%. Mean net soil erosion rates were approximately 4 t ha⁻¹ per annum (median value 0.41 t ha⁻¹ per annum) and associated mean P losses 3.4 kg ha⁻¹.