Wesley W. Wallender

Sustainability of irrigated agriculture in the San Joaquin Valley, California

The sustainability of irrigated agriculture in many arid and semiarid areas of the world is at risk because of a combination of several interrelated factors, including lack of fresh water, lack of drainage, the presence of high water tables, and salinization of soil and groundwater resources. Nowhere in the United States are these issues more apparent than in the San Joaquin Valley of California. A solid understanding of salinization processes at regional spatial and decadal time scales is required to evaluate the sustainability of irrigated agriculture. A hydro-salinity model was developed to integrate subsurface hydrology with reactive salt transport for a 1,400-km2 study area in the San Joaquin Valley. The model was used to reconstruct historical changes in salt storage by irrigated agriculture over the past 60 years. We show that patterns in soil and groundwater salinity were caused by spatial variations in soil hydrology, the change from local groundwater to snowmelt water as the main irrigation water supply, and by occasional droughts. Gypsum dissolution was a critical component of the regional salt balance. Although results show that the total salt input and output were about equal for the past 20 years, the model also predicts salinization of the deeper aquifers, thereby questioning the sustainability of irrigated agriculture.

Reclaiming freshwater sustainability in the Cadillac Desert

Increasing human appropriation of freshwater resources presents a tangible limit to the sustainability of cities, agriculture, and ecosystems in the western United States. Marc Reisner tackles this theme in his 1986 classic Cadillac Desert: The American West and Its Disappearing Water. Reisners analysis paints a portrait of region-wide hydrologic dysfunction in the western United States, suggesting that the storage capacity of reservoirs will be impaired by sediment infilling, croplands will be rendered infertile by salt, and water scarcity will pit growing desert cities against agribusiness in the face of dwindling water resources. Here we evaluate these claims using the best available data and scientific tools. Our analysis provides strong scientific support for many of Reisners claims, except the notion that reservoir storage is imminently threatened by sediment. More broadly, we estimate that the equivalent of nearly 76% of streamflow in the Cadillac Desert region is currently appropriated by humans, and this figure could rise to nearly 86% under a doubling of the regions population. Thus, Reisners incisive journalism led him to the same conclusions as those rendered by copious data, modern scientific tools, and the application of a more genuine scientific method. We close with a prospectus for reclaiming freshwater sustainability in the Cadillac Desert, including a suite of recommendations for reducing region-wide human appropriation of streamflow to a target level of 60%.

PLOTSIZE AND SAMPLE NUMBER FOR NEUTRON PROBE MEASUREMENTS IN SMALL FIELD TRIALS

Soil water storage over a 2.85-m soil depth was measured from 200 aluminum access pipes, separated by 0.3 m in both directions, in a 1.2 × 15.0-m plot in dry and wet periods during 1988 and 1989. The objective was to determine the minimum plot size and number of soil water content measurements if measured with a neutron probe for small field trials. The minimum plot size representing the 15-m plot was found to be dependent on water storage variance and on the distance over which soil water storage measurements were spatially correlated. We concluded that a plot length of 5 m was needed to represent the mean and variance of the 15-m plot. Bootstrapping and temporal stability analysis were used to estimate the minimum number of observation tubes required to estimate the mean and variance of 1.2 × 5.0-m plots. Bootstrapping showed that at least 10 soil water storage measurements were required in the 5-m plot. Soil water storage distribution within the plot was found to be highly stable in time, especially for individual soil layers. Using temporal stability analysis, the number of required access pipes needed to estimate a plot-average soil water storage was further reduced to three. However, the variance of soil water storage was not conserved while reducing the number of measurement locations. We propose that a field study with small field trials should start with the maximum feasible plot size and number of measurement locations. In this initial phase, statistical techniques as proposed in this study can then be applied to reduce the required number of observations, using predetermined error limits.

INFILTRATION AND SOIL WATER STORAGE UNDER WINTER COVER CROPPING IN CALIFORNIA’S SACRAMENTO VALLEY

Winter cover cropping on agricultural fields may improve rainfall infiltration and enhance soil water storage in areas such as California’s Sacramento Valley, where the majority of precipitation occurs in the winter over a relatively short period of time in a series of heavy rainfall events. Enhanced soil water storage within the root zone on cover–cropped fields may benefit a grower by reducing the demand for surface water deliveries to meet the irrigation needs of subsequent crops. A study was conducted in the winters of 1998–1999 and 1999–2000 to determine a field’s ability to conserve water for subsequent crops and to evaluate the effects of soil physical conditions on the water balance for three 4–year rotation farming systems within the Sustainable Agriculture Farming Systems (SAFS) Project at the University of California, Davis. Rainfall, runoff, and soil water content data was collected on two treatments using a winter cover crop and one treatment maintained fallow during the winter. Runoff and soil water content measurements were significantly affected by farming systems. Cumulative event runoff from 10.67 m 2 infiltration test areas was consistently higher on the fallow treatment than on the cover–cropped treatments. Winter 1999–2000 field water content measurements from 0–1.05 m depth were significantly higher in the cover–cropped systems than in the fallow treatment after field capacity had been reached. A hydrologic model was developed using the measured data and lysimeter data for evaporation and evapotranspiration to track daily water budget components (i.e., runoff, infiltration, evaporation, evapotranspiration, and soil water storage) and to assess changes in surface hydraulic conductivity. Model simulations showed that optimized hydraulic conductivity decreased for all treatments with successive runoff, but was less pronounced in cover–cropped plots. The study indicated that cover cropping can improve soil water storage for subsequent crops if the cover crop is destroyed before the additional soil water is lost as evapotranspiration.

Spatial Variability of Infiltration in Furrows

ABSTRACT THE mean and spatial variability of infiltration measured with rings, blocked furrows with stagnant ponded water, blocked furrows with flowing water, and blocked furrows with surge flow were evaluated. Infiltration is generally greater with water flowing in blocked furrows than stagnant tests, especially on cracked soil. Spatial variability of cumulative infiltration is greater than for quasi-steady infiltration and the distance over which samples are spatially related is also greater for cumulative infiltration. Blocked furrow measurements with flowing water are preferred to the other stagnant tests because they more closely duplicate conditions under furrow irrigation. Spatial variability of infiltration characteristics should be included in evaluating the performance of furrow irrigation systems. The autocorrelogram is introduced as a tool to determine distance between samples to avoid spatial correlation and thus get the maximum new information regarding variability from sampling. A similar tool, the crosscorrelogram shows promise for estimating blocked furrow intake from ring infiltration test.

Estimation of in situ unsaturated soil hydraulic functions from scaled cumulative drainage data

Simulation of water flow and transport processes in soils rely on field representative soil hydraulic functions. The linear variability concept in combination with the inverse technique was used to estimate in situ soil hydraulic properties in a 32-ha field. Measured cumulative drainage curves were scaled yielding scaling factors. Subsequently, the drainage and moisture content distribution of the scaled reference profile were input to a numerical model to optimize the soil water retention and hydraulic conductivity curves for the reference soil profile by inverse solution of the scaled Richards equation. Field hydraulic functions for each location were computed from the reference curves and scaling factors. In addition, undisturbed soil cores taken from 0.3-m and 0.6-m depths at 44 locations were used to determine soil texture, and soil water retention and hydraulic conductivity curves in the laboratory using the multistep outflow technique. These hydraulic functions were scaled using the simultaneous scaling technique. The reference field hydraulic functions compared well with those determined from the soil cores taken from the 0.6-m depth. In situ saturated hydraulic conductivity variability was one order of magnitude less than that of the soil cores.

Effect of Estuarine Wetland Degradation on Transport of Toxoplasma gondii Surrogates from Land to Sea

ABSTRACT The flux of terrestrially derived pathogens to coastal waters presents a significant health risk to marine wildlife, as well as to humans who utilize the nearshore for recreation and seafood harvest. Anthropogenic changes in natural habitats may result in increased transmission of zoonotic pathogens to coastal waters. The objective of our work was to evaluate how human-caused alterations of coastal landscapes in California affect the transport of Toxoplasma gondii to estuarine waters. Toxoplasma gondii is a protozoan parasite that is excreted in the feces of infected felids and is thought to reach coastal waters in contaminated runoff. This zoonotic pathogen causes waterborne toxoplasmosis in humans and is a significant cause of death in threatened California sea otters. Surrogate particles that mimic the behavior of T. gondii oocysts in water were released in transport studies to evaluate if the loss of estuarine wetlands is contributing to an increased flux of oocysts into coastal waters. Compared to vegetated sites, more surrogates were recovered from unvegetated mudflat habitats, which represent degraded wetlands. Specifically, in Elkhorn Slough, where a large proportion of otters are infected with T. gondii, erosion of 36% of vegetated wetlands to mudflats may increase the flux of oocysts by more than 2 orders of magnitude. Total degradation of wetlands may result in increased Toxoplasma transport of 6 orders of magnitude or more. Destruction of wetland habitats along central coastal California may thus facilitate pathogen pollution in coastal waters with detrimental health impacts to wildlife and humans.

Furrow Hydraulic Characteristics and Infiltration

ABSTRACT THE influence of temporally varying flow rate and surface depth on measured infiltration, furrow roughness, and geometry were compared using classical and regionalized statistical theory. Flowing conditions, rather than stagnant water, enhanced intake on cracked Yolo clay loam whereas rapidly increasing surface flow depth enhanced infiltration on the same soil with fewer cracks. There is a significant cross-correlation between wetted perimeter and infiltration where cracks and holes do not dominate infiltration. The measurements were not correlated for distances of 8 m or more, however. Roughness decreased and the furrow geometry became more hydraulically efficient during irrigation. Displacement tests and cross-section measurements suggest that soil swelling may inflate estimates of deposition in furrows. Dewatering after the first surge in surge irrigation decreased deposition.

Droplet Size Distribution and Water Application with Low-Pressure Sprinklers

ABSTRACT SPRINKLERS with circular and noncircular, low-pressure, nozzles were tested in order to determine droplet diameters and water application uniformity. Volume weighted mean droplet diameter was greater for noncircular nozzles at a given distance from the sprinkler, but the maximum droplet diameter was greater for circular nozzles near the perimeter of the wetted pattern. This was possible because wetted diameter was greater for circular nozzles. Although droplet size is inversely related to jet velocity for circular and noncircular nozzles, the relations are not identical and the mechanism of droplet formation from noncircular nozzles should be further investigated. The soil damage hazard from large droplets is further compounded in the case of circular nozzles at low pressures due to high application rates near the perimeter. Added equipment cost, associated with retrofitting existing medium or high pressure sprinkler systems, should be offset by energy savings. Equipment cost is greater because sprinkler spacing generally decreases when low-pressure sprinklers are used.

One-point Method for Estimating Furrow Infiltration

A quick method to estimate parameters of the Philip infiltration function from advance time to the field end, flow rate and flow area were developed using a volume balance principle. By assuming a power advance exponent of 1/2, the volume balance equation resulted in a closed-form integral solution and the infiltration parameters were found from advance to the field end only rather than from two or more points of advance along the field. Average infiltration for a furrow was estimated from intake opportunity time (calculated from advance and recession times) and Philip’s (1957) infiltration function. Model results were compared with more time-consuming volume balance (Philip and Farrell, 1964), neutron probe, and infiltrometer methods. Estimates of furrow infiltration using the one-point method agreed with the standard which was considered to be between neutron probe and field water application estimates. The Philip equation fitted to the infiltrometer data overpredicted for the first irrigation because preferential flow occurred along the perimeter of the bypass infiltrometer. For the latter irrigations infiltration was underpredicted because it was not possible to measure infiltration with the flow-through infiltrometer during the first 10 to 20 min of irrigation when the rate was high. The Philip and Farrell method matched the neutron probe results, and both underpredicted furrow infiltration. For the one-point method, sorptivity, S, was greater and parameter A was lower than predicted from infiltrometer measurements and the Philip and Farrell method, but the parameters compensated for one another and predicted infiltration agreed with the standard. The one-point method can be used to estimate average infiltration of an individual furrow from only advance time to the field end, flow rate, and flow area. Although effective for estimating average infiltration on a heterogeneous field, the one-point method and other volume balance methods did not accurately predict the distribution of water along the furrows with a dramatic change in infiltration properties.