Agricultural runoff and nutrient loading
The Central Valley is one of the most productive agricultural regions in the world, and that productivity comes with significant nutrient loading to local waterbodies. Dairy operations, which are concentrated in the southern San Joaquin Valley around Tulare and Kings Counties, produce manure that is applied to cropland as fertilizer. When irrigation water or rainfall washes across these fields, it carries dissolved nitrogen and phosphorus into drainage ditches, canals, and eventually into ponds, lakes, and rivers.
Row crop agriculture and orchard operations throughout the valley also contribute nutrients through fertilizer application. Irrigation return flows, water that runs off the tail end of irrigated fields, carry dissolved fertilizer residues that can significantly elevate nutrient concentrations in receiving waterbodies. In some areas, these return flows are the primary water source for downstream ponds and reservoirs.
The scale of nutrient loading in the Central Valley is different from other California regions. While urban areas contribute nutrients through landscaping runoff and stormwater, the Central Valley's agricultural operations apply fertilizer across millions of acres, creating a nutrient supply that dwarfs urban sources in most watersheds.
- Dairy operations in Tulare and Kings Counties are a major source of nitrogen and phosphorus loading.
- Irrigation return flows carry dissolved fertilizer residues into downstream ponds and waterways.
- Agricultural nutrient loading in the Central Valley far exceeds typical urban nutrient sources.
- Manure application to cropland contributes both nitrogen and phosphorus to runoff during irrigation events.
Extreme heat and its effects on waterbodies
The Central Valley experiences some of California's most extreme summer temperatures, with areas like Fresno, Bakersfield, and Stockton regularly exceeding 100°F from June through September. These sustained high temperatures create challenging conditions for waterbody management. Water temperatures in shallow ponds can reach the upper 80s or even low 90s, severely reducing dissolved oxygen capacity and accelerating organic decomposition rates.
Hot summers also mean high evaporation rates. Ponds and small reservoirs without supplemental water inputs can lose significant volume during the summer months, concentrating nutrients and contaminants as the water level drops. This evaporative concentration effect can turn a moderately nutrient-rich pond into a hyper-eutrophic system by late summer, even without any additional nutrient inputs.
Don Pedro Reservoir and New Melones Reservoir, two of the Central Valley's major foothill storage facilities, experience substantial drawdowns during dry years as water is released for irrigation and downstream commitments. These water level fluctuations expose large areas of shoreline sediment, which can dry, crack, and release stored phosphorus when re-wetted by fall rains or reservoir refilling.
- Summer air temperatures regularly exceed 100°F in the valley floor from June through September.
- Water temperatures in shallow ponds can reach the upper 80s or low 90s during peak summer.
- Evaporative concentration effects can turn moderately nutrient-rich ponds into hyper-eutrophic systems.
- Don Pedro and New Melones Reservoirs experience significant drawdowns during drought years.
Central Valley Regional Water Board and irrigated lands programs
The Central Valley Regional Water Quality Control Board (Region 5) is the largest of California's nine regional boards by area, covering the Sacramento and San Joaquin River watersheds and the Tulare Lake Basin. The Board administers several programs directly relevant to pond and lake owners in agricultural settings, most notably the Irrigated Lands Regulatory Program (ILRP).
The ILRP requires growers and landowners to monitor and report on the quality of water leaving their irrigated lands. Properties that discharge irrigation return flows or tailwater to surface waters are generally required to participate in a coalition monitoring group or obtain individual waste discharge requirements. Ponds that receive and discharge these agricultural return flows may be considered part of the discharge pathway and subject to regulatory requirements.
For ranch and farm pond owners, understanding your property's relationship to the ILRP is important. If your pond receives irrigation tailwater and eventually drains to a surface waterway, the water quality in your pond may be part of the monitoring and reporting that your coalition or individual permit requires. This can actually work in your favor: effective pond management that reduces nutrient concentrations before discharge can help meet regulatory requirements.
- The Central Valley Regional Board (Region 5) is the largest regional board by area in California.
- The Irrigated Lands Regulatory Program requires monitoring of water quality from irrigated agricultural lands.
- Farm ponds receiving irrigation tailwater may be part of regulated discharge pathways.
- Effective pond management can help meet ILRP monitoring and reporting requirements.