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California has the largest agricultural economy in the U.S. and is noted worldwide for its high productivity, quality and efficiency in producing fresh market and value-added food. In spite of a Mediterranean climate that is dry during the growing season, irrigation allows the state to produce high yields. Because of its low capital investment, furrow irrigation is the most commonly used irrigation system, however, the agricultural practices used to produce the quality products are affecting the sustainability of crop production systems.
One of the experiments in the SAFS fields at UC Davisí Russell Ranch Sustainable Agriculture Facility is looking at the effect of temperature on runoff, and the amount of dissolved organic carbon (DOC) that appears in the runoff.
The runoff from furrow-irrigated fields enters surface waters and potentially causes downstream water quality impairment. Since surface water is the major source of drinking water for over two-thirds of Californians, measures need to be taken to reduce the impact of irrigation runoff. Organic materials, which produce DOC in runoff, are responsible for degraded taste, odor and color of water and the formation of carcinogenic disinfection byproducts during water treatment. Mitigation of DOC in runoff has been recognized as a critical part of the irrigation water management.
One of the important methods of controlling surface water pollution is to use best management practices (BMPs) in agricultural fields, which reduce organic materials such as DOC in the runoff. BMPs such as conservation tillage (CT) and use of cover crops (CC) can reduce runoff by promoting water infiltration into soil. During an irrigation event, DOC can be released through the leaching of soluble components of residue, and the diffusion of soil organic matter in running water. The DOC concentration increases with higher temperatures, changes in farming practices, and the amount and quality of water flow over and through the soil. Increasing temperature can increase microbial activity and associated decomposition of crop residues and may enhance the DOC concentration in runoff water. The objective of our research is to determine the effect of temperature on irrigation runoff DOC concentration from furrow irrigation system under various conservation practices.
|Table 1: Field condition before summer 2007|
|Season||Cover crops (CC)||No-till (NT)||Standard tillage (ST)|
|Sunflower grown & harvested||Sunflower grown & harvested||Sunflower grown & harvested|
|Fall||Sunflower stalks left over the surface||Sunflower stalks left over the surface||Sunflower stalks incorporated|
|Cover crop (wheat) grown||Bare soil||Bare soil|
|Spring||No till||No till||Tilled|
|Summer||42% residue cover||32% residue cover||11% residue cover|
A furrow irrigated field of 366 m (1200 feet) in length with three treatments, cover crop (CC), no-till (NT) and standard tillage (ST) with three replications was used in our study. The average measured surface residue cover was 42% for CC, 32% for NT and 11% ST at the beginning of irrigation. Table 1 shows the sequence of various cultural operations used for residue management on these plots.
Irrigation water was delivered to the treatments at a typical inflow of 0.054 m3 s-1 (850 GPM). The outflow from each treatment was collected using ISCO auto-samplers at an interval of 2.5 hours during the first 24 hours and an interval of five hours thereafter (Fig. 1). ISCO auto-samplers also measured the runoff rate to determine the amount of water leaving the field. The DOC was analyzed using UV-persulfate oxidation and its concentration combined with runoff and temperature was used to analyze the effect of temperature on runoff and DOC concentration in the different treatments.
The runoff rate generally decreased with increasing temperature for all treatments (Figure 2). The rise in temperature may increase water evaporation both from the field surface and from the ponding water in furrows. However, the evaporation was negligible compared to the inflow rate. Water draw from the irrigation canal by neighboring growers may also have reduced water input to our fields. The runoff rate ranged from 0.0032 to 0.0002 m3/s for CC, 0.0054 to 0.0002 m3/s for NT and 0.0021 to 0.0002 m3/s for ST, indicating that NT has more runoff than CC and ST. The CC treatment increases soil aggregation and thus infiltration, which decreases runoff, but the lack of tillage did not have the same effect in the NT treatment as seen in other studies.
The DOC concentration was highest at the beginning of runoff and gradually decreased (Figure 3). At the beginning of irrigation, dry soil has the capacity to infiltrate more water. Thus the water entering the furrows has more residence time to interact with soil and residue producing more DOC. The DOC mixes with the soil water and is convected to the tailend as an initial flush resulting in maximum DOC concentration at the beginning of runoff. The DOC decreased over the irrigation event.
The results also show that DOC dissolution in soils increases as temperatures rises. Higher temperatures increase microbial activity, and accelerate release of DOC. The series of DOC peaks corresponded to diurnal temperature swings. The increase in DOC concentration for every 1oC increases in temperature varied between 0.3 to 0.08 mg/l for CC, 0.4 to 0.05 mg/l for NT and 0.2 to 0.08 mg/l for ST. The DOC concentration varied between 8 to 3 mg/l, 12 to 3 mg/l and 6 to 2 mg/l for CC, NT and ST treatments. Hence, NT releases more DOC followed by CC then ST.
The CC treatment releases less runoff and DOC followed by ST then NT. The results show that these treatments have varying effects on DOC export from agriculture fields. This information is important in devising strategies to reduce DOC export and to maintain irrigation efficiency