Water for irrigation is a major limitation to agricultural production in many parts of the world. Use of waters with elevated levels of salinity is one likely option to meet the supply of increased ...demands. The sources of these waters include drainage water generated by irrigated agriculture, municipal wastewater, and poor quality groundwater. Soil salinity leaching requirements that were established several decades ago were based on steady-state conditions. Recently transient-state models have been developed that potentially can more correctly predict the dynamics of the chemical–physical–biological interactions in an agricultural system. The University of California Center for Water Resources appointed a workgroup to review the development of steady-state analyses and transient-state models, and to determine whether the current recommended guidelines for leaching requirement based on steady-state analyses need to be revised. The workgroup concludes that the present guidelines overestimate the leaching requirement and the negative consequences of irrigating with saline waters. This error is particularly large at low leaching fractions. This is a fortuitous finding because irrigating to achieve low leaching fractions provides a more efficient use of limited water supplies.
A general tend exists toward increasing the salt concentration of surface and ground waters in the arid and semi-arid regions of the world. Quantitative knowledge on the interaction between ...irrigation management and crop yield is becoming increasingly important as water salinities increase. This information is important to the farmer, water resource managers, and to regulatory agencies that might be considering imposing saline water quality standards for streams specifically designed to be protective of agricultural production. The initial guidelines for managing saline irrigation waters, which are still extensively used, were based on steady-state analyses. Subsequently, with the increased knowledge of the physical–chemical–biological interactions that occur in the soil-water–plant matrix, and the advent of high-speed computers, models have been developed that take into account the dynamic interactions. A comparison between two steady-state models and a transient-state model for the calculated ratio of applied saline water to potential evapotranspiration that produced a specific relative yield of corn (
Zea mays L.) revealed that a much lower water application is prescribed by the transient-state model.
The effects of rain can be quantified by the transient-state models, and can be approximated by calculating the weighted average water salinity of the rainfall and irrigation waters. The steady-state analyses generally over predict the negative consequences of irrigating with saline waters. Irrigation guidelines based on steady-state analyses should be revised using more advanced knowledge.
► ENVIRO-GRO, HYDRUS, SALTMED, SWAP and UNSATCHEM were used to simulate forage corn yields. ► Forage corn yields were compared where 0.5
≤
ECiw
≤
6
dS/m and weekly irrigation ranged from 0.9 to 1.3 ...times potential crop evapotranspiration (Kirr). ► Forage corn yields for SALTMED were lower than for the other models for all conditions. ► For Kirr
≥
1.1 and 0.5
≤
ECiw
≤
2
dS/m, forage yields for ENVIRO-GRO were higher because it includes plant-based compensation. ► Forage yields for UNSATCHEM were higher than for HYDRUS and SWAP because of the effects of chemical composition on osmotic stress. ► For ECiw
≤
3
dS/m, the relative yields for ENVIRO-GRO, HYDRUS, SWAP and UNSATCHEM are within 7% or less for all Kirr values.
Transient-state models that account for continually changing salinity and matric stress on crop yields have been developed by several research groups. The objective of this research was to compare the simulated yields of forage corn obtained from a common set of soil and water conditions for ENVIRO-GRO, HYDRUS, SALTMED, SWAP and UNSATCHEM. The physical and hydraulic properties of Panoche clay loam were used. The amounts of water applied weekly, based on the climatic conditions in the San Joaquin Valley of California, ranged from 0.9 to 1.3 times (Kirr) the potential evapotranspiration (PET) of corn. The salinity of the applied water (ECiw) ranged from 0.5 to 6
dS/m which brackets the threshold soil–water salinity of forage corn (
Zea mays L.) of 3.6
dS/m. The model simulations were run for sufficient back-to-back crop seasons to establish transient matric and osmotic conditions within the root zone that did not change from one crop season to the next, a quasi steady-state condition. SALTMED simulated lower relative yields (RY) than the other models for all combinations of Kirr and ECiw. For the other models, RY values were similar (within about 7% or less) for ECiw
≤
3
dS/m for all Kirr values. Plots of RY versus ECiw for HYDRUS, SWAP, and UNSATCHEM approximately paralleled each other except that UNSATCHEM produced higher values. ENVIRO-GRO produced the highest RY where Kirr
≥
1.1 and ECiw
≤
2.0
dS/m but decreased more rapidly for greater ECiw. ENVIRO-GRO has plant-based compensation which allows water uptake to meet PET as long as any portion of the root zone is not exposed to matric or osmotic stress that exceed threshold levels. This compensation factor produced higher RY at the lower ECiw values. More rapid decrease in RY with increasing values of ECiw simulated by ENVIRO-GRO is attributed to the assumption that the osmotic and matric stresses are additive, whereas the others assume that they were multiplicative. All the models except UNSATCHEM assume a constant relationship between EC and salt concentration in solution. UNSATCHEM takes into account the effects of the ionic composition and ion concentration on osmotic potential, resulting in higher RY values obtained with this model. Since the chemical composition of irrigation waters are all unique, this aspect of UNSATCHEM poses an important capability in the assessment of osmotic effects on crop yields. We conclude the models provide a valuable resource to assess the utility of moderately saline irrigation waters, for a broad range of transient conditions which include variable crops, precipitation, irrigation water management, and irrigation water salinity. We also highly recommend their use to assess the results obtained in experiments that focus on the responses of crop growth and yield to transient changes in soil water content and salinity.
Measurement techniques that quantify the degree of soil water repellency are important for research and for the communication of research findings. The water drop penetration time (WDPT) is a ...commonly used measurement. If a water drop does not enter the soil spontaneously, the soil-water contact angle is greater than 90 degrees and the soil is considered to be water repellent. The time for the drop to enter the soil (WDPT) provides an indication of the stability of the repellency. The liquid-air surface tension of an aqueous ethanol concentration series that enters the soil in approximately 5 s is identified as the ninety degree (ND) surface tension, gamma(ND), of the soil. The gamma(ND) number can be used to calculate the solid-air surface tension, gamma(s), by gamma(s) = gamma(ND)/4. The water-soil contact angle can also be calculated from the gamma(s) value by the relationship cos theta = (gamma(ND)/gamma(w))1/2 - 1, where theta is the contact angle and gamma(w) the water-air surface tension. The water entry pressure, h(p), which is a function of both the soil water repellency and pore size, is an important parameter for predicting infiltration and the stability of water flow in the field. Measurements of WDPT, gamma(ND), and h(p) provide a complete characterization of the degree of water repellency.
Knowing the sorptive behavior of anionic polyacrylamide (PAM) by soils is useful in predicting the appropriate application rate, depth of effective treatment, and its mobility in soils. Sorption ...isotherms of PAM by soil materials, six natural soils, and their subsamples with partial organic matter (OM) removed by H2O2 oxidization under different dissolved salt concentrations were examined. The PAM sorption isotherms can be well described by the Langmuir equation. Soil texture, OM content, and dissolved salts (a combined contribution of soil salinity and irrigation water quality) influenced the extent of PAM sorption. Soils with high clay or silt content and low OM content had a high sorptive affinity for anionic PAM. The amount of PAM saturation sorption increased significantly as the total dissolved salts (TDS) increased. Divalent cations Ca2+ and Mg2+ were about 28 times more effective in enhancing PAM sorption than monovalent cations Na+ and K+, mainly because of their stronger charge screening ability. The effectiveness of cation enhancement on PAM sorption varied with soil texture and was greater in fine soils than in sandy soils. Organic matter had a negative effect on PAM sorption. Soil samples after the removal of partial OM adsorbed more PAM than natural soils. The negative effect of OM on PAM sorption was attributed to the reduction of accessible sorption sites by cementing inorganic soil components to form aggregates and to the enhancement of electrostatic repulsion between PAM and soil surface by its negatively charged functional groups.
Soil quality is a concept that has deeply divided the soil science community. It has been institutionalized and advocated
without full consideration of concept weaknesses and contradictions. Our ...paper highlights its disfunctional definition, flawed
approach to quantification, and failure to integrate simultaneous functions, which often require contradictory soil properties
and/or management. While the concept arose from a call to protect the environment and sustain the soil resource, soil quality
indexing as implemented may actually impair some soil functions, environmental quality, or other societal priorities. We offer
the alternative view that emphasis on known principles of soil management is a better expenditure of limited resources for
soil stewardship than developing and deploying subjective indices which fail to integrate across the necessary spectrum of
management outcomes. If the soil quality concept is retained, we suggest precisely specifying soil use, not function or capacity,
as the criteria for attribute evaluation. Emphasis should be directed toward using available technical information to motivate
and educate farmers on management practices that optimize the combined goals of high crop production, low environmental degradation,
and a sustained resource.
Soil organic matter can be divided into solid (SOM) and water‐dissolved (DOM) fractions; both of which can associate with herbicides. The objective was to examine the effects of DOM in applied ...solutions on atrazine adsorption in soil containing various SOM contents. A sandy loam with low (0.14%, Soil L), medium (1.2%, Soil M), or high (6.7%, Soil H) total organic matter, and artificial soil (Soil A) with no organic matter were used. These soils were treated with several different atrazine concentrations, dried, and extracted with water; the DOM concentrations in these extracts were 0, 10.1, 41.1, and 156.5 mg L−1, respectively. These extracts and the untreated soils were used in batch experiment. The distribution coefficients (Kd) for atrazine of Soils L, M, and H under mixing with Soil A extract were 0.19, 0.52, 2.51, respectively, indicating that the higher the SOM content, the higher the atrazine affinity to the soil solid phase. The effects of the DOM concentration on atrazine adsorption were estimated from the ratios (Rd) between the Kd for applied solution with no DOM and those for applied solutions containing DOM. Rd > 1 indicates that the DOM decreases the herbicide adsorption. The Rd values for Soils L, M, and H, each soil mixed with its extract, were 2.68, 1.93, and 1.07, respectively. In contrast, in the case where the soil was mixed with a solution that contained a higher DOM concentration than the soil extract, an increase in the DOM concentration in the applied solution decreased the Rd value. This was probably because of adsorption of DOM‐atrazine complexes on the SOM.
Policy and management plans can be enhanced through effective communication between researchers and decision makers. Differences in understanding can come from differences in professional cultures. ...Scientists deal with facts, proof and incremental progress whereas the decision makers are often faced with perceptions, emotions and deadlines. A case study is presented illustrating the interaction between the political system and science on a water management issue. Irrigation projects in the western San Joaquin Valley of California lead to a situation requiring subsurface drainage and disposal of the drainage water. The original plan was to discharge the drainage water in the Suisun Bay east of the San Francisco Bay. Severe damage to birds associated with selenium in the water led to a reevaluation of irrigation and drainage management options. Federal and state agencies cooperated to establish a San Joaquin Valley Drainage Program (SJVDP) which was to develop plans for solving the problem. Discharge to the Bay was politically eliminated as an option for evaluation, an action criticized by a National Research Council Committee as not being scientifically based. The SJVDP published a Management Plan in 1990 which contained proposals viewed by the scientific community as not necessarily incorrect but not completely justified based on the scientific knowledge at the time. A segment of the Citizens Advisory Committee that was part of the SJVDP consisting of representatives from the interest groups viewed the Management Plan as a negotiated agreement between the environmental and agricultural interests. Presently, an Activity Plan exists, consisting of technical committees to evaluate the current technical and economic evaluation of the management options proposed in the Management Plan. This case study illustrates that factors other than scientific facts have bearing on decisions. Successful management plans must be technically sound, economically viable and socially acceptable. The scientific community needs to evaluate its role in the policy making arena and to focus research on questions of greatest value to decision makers, as well as to scientific peers.
Development of a multicomponent model for crop yield and potential groundwater degradation applicable for irrigated agriculture is important. The ENVIRO-GRO model was developed to simulate (i) water, ...salt, and N movement through soil with a growing plant; (ii) plant response to matric potential, salinity, and N stresses; (iii) drainage and salt and N leaching; and (iv) cumulatively relative transpiration and relative N uptake, and consequent crop relative yield. This model does not account for denitrification. The utility of the model was illustrated by simulating the effects of irrigation amount, irrigation salinity, and N application on yield and N leaching. The results demonstrated the effects of complex interactions and feedback mechanisms in the plant-soil-water-salinity-N system. Factors leading to reduced plant growth caused lower transpiration, which created more leaching of salts and N. The simulated interactions are consistent with observed behaviors. Evaluation was done by comparing simulated results with published results of an experiment that had N application rates of 0, 90, 180, and 360 kg N ha-1 and water application rates of 21, 63, and 105 cm. Agreement between simulated and observed corn (Zea mays L.) relative yield and total N uptake was generally good. The difference between mean observed and predicted values was 0.06 for corn relative yield and 1.34 kg N ha-1 for total N uptake. Linear regression analyses revealed excellent agreement results for the 63-cm irrigation and a tendency for the model to overpredict results for the 21-cm irrigation and underpredict results for the 105-cm treatment
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