Testing a two-scale focused conservation strategy for reducing phosphorus and sediment loads from agricultural watersheds

Abstract

This study tested a focused strategy for reducing phosphorus (P) and sediment loads in agricultural streams. The strategy involved selecting small watersheds identified as likely to respond relatively quickly, and then focusing conservation practices on high-contributing fields within those watersheds. Two 5,000 ha (12,360 ac) watersheds in the Driftless Area of south central Wisconsin, previously ranked in the top 6% of similarly sized Wisconsin watersheds for expected responsiveness to conservation efforts to reduce high P and sediment loads, were chosen for the study. The stream outlets from both watersheds were monitored from October of 2006 through September of 2016 for streamflow and concentrations of sediment, total P, and, beginning in October of 2009, total dissolved P. Fields and pastures having the highest potential P delivery to the streams in each watershed were identified using the Wisconsin P Index (Good et al. 2012). After three years of baseline monitoring (2006 to 2009), farmers implemented both field-and farm-based conservation practices in one watershed (treatment) as a means to reduce sediment and P inputs to the stream from the highest contributing areas, whereas there were no out-of-the-ordinary conservation efforts in the second watershed (control). Implementation occurred primarily in 2011 and 2012. In the four years following implementation of conservation practices (2013 through 2016), there was a statistically significant reduction in storm-event suspended sediment loads in the treatment watershed compared to the control watershed when the ground was not frozen (p = 0.047). While there was an apparent reduction in year-round suspended sediment event loads, it was not statistically significant at the 95% confidence level (p = 0.15). Total P loads were significantly reduced for runoff events (p < 0.01) with a median reduction of 50%. Total P and total dissolved P concentrations for low-flow conditions were also significantly reduced (p < 0.01) compared to the control watershed. This study demonstrated that a strategy that first identifies watersheds likely to respond to conservation efforts and then focuses implementation on relatively high-contributing fields within those watersheds can be successful in reducing stream P concentrations and loads.

Seasonal Variation in Sediment and Phosphorus Yields in Four Wisconsin Agricultural Watersheds

“…4c and 4d, Table 4) but had the lowest mean soil test P (Table 1). It is also interesting that agricultural land use inventories in these watersheds showed no winter manure applications or livestock access to streams, whereas both of these were present in P1 and P2 (Carvin et al, 2018; C. Ertman, Sheboygan County Land Conservation Department, written communication, 2018).…”

“…The USGS installed streamflow‐gaging and water‐quality monitoring stations at the outlets of P1 (USGS Station 05432927) and P2 (USGS Station 05432695) in fall 2006 and at the outlets of S1 (USGS Station 040857005) and S2 (USGS Station 040854592) in summer 2011. Gaging, sampling, and analysis protocols are described in Carvin et al (2018) for year‐round water sample collection during low‐flow periods (baseflow) and during rain‐on‐snow, snowmelt, and rain events from October 2006 through September 2016 in P1 and P2 and from October 2011 through September 2016 in S1 and S2. Streamflow (discharge), SS, and TP samples were collected for the entire monitoring periods for all watersheds.…”

“…This report summarizes monitoring data on year‐round sediment and P loads for four agricultural watersheds (Fig. 1) collected for paired watershed projects (Ertman, 2017; Carvin et al, 2018). Research in one of the watershed pairs suggested a frozen versus nonfrozen seasonal difference in SS load response to conservation practice implementation (Carvin et al, 2018).…”

“…1) collected for paired watershed projects (Ertman, 2017; Carvin et al, 2018). Research in one of the watershed pairs suggested a frozen versus nonfrozen seasonal difference in SS load response to conservation practice implementation (Carvin et al, 2018). An earlier conservation project in one of the other watersheds also indicated a seasonal (vegetative versus nonvegetative) difference in response to watershed‐wide efforts to reduce stream storm loads of SS and TP (Corsi et al, 2005).…”

“…The first two study watersheds, Pleasant Valley (P1) and Ridgeway (P2), are in the unglaciated region of Wisconsin in the Pecatonica River basin and have been previously described (Carvin et al, 2018). The second set of watersheds, Otter Creek (S1) and Fisher Creek (S2), are in the previously glaciated region of Wisconsin in the Sheboygan River basin.…”