Predicting Sea Level Rise Impacts on Ag Production: Difference between revisions

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'''This project will investigate sea level rise impacts on saltwater intrusion and explore methodologies to detect and quantify land subsidence and river channel aggradation changes that may impact agricultural viability now and in future climate conditions; this work will inform the [[Snohomish Conservation District]]'s [[Snohomish Agriculture Resilience Plan]] and is funded by the The [[ESRP]] Learning Program.'''<br><br>
'''This project will investigate sea level rise impacts on saltwater intrusion and explore methodologies to detect and quantify land subsidence and river channel aggradation changes that may impact agricultural viability now and in future climate conditions; this work will inform the [[Snohomish Conservation District]]'s [[Snohomish Agriculture Resilience Plan]] and is funded by the The [[ESRP]] Learning Program.'''<br><br>


The [[Snohomish Conservation District]] will lead a collaborative effort with the [[Snohomish Sustainable Lands Strategy]], UW Climate Impacts Group, Cardno, and NOAA to evaluate how climate change, sea level rise, and vertical land movement are likely to affect agricultural systems in the tidally influenced Lower Snohomish and Lower Stillaguamish river floodplains. This information is anticipated to support estuary restoration as an appropriate response to protecting regional agricultural viability under seal level rise by describing the value of a strategic retreat from vulnerable low-lying areas, and encouraging a renewed focus on protecting agricultural lands and irrigation water resources at higher elevations.   
The [[Snohomish Conservation District]] led a collaborative effort with the [[Snohomish Sustainable Lands Strategy]], UW Climate Impacts Group, Cardno, and NOAA to evaluate how climate change, sea level rise, and vertical land movement are likely to affect agricultural systems in the tidally influenced Lower Snohomish and Lower Stillaguamish river floodplains. This information supports estuary restoration as an appropriate response to protecting regional agricultural viability under sea level rise by describing the value of a strategic retreat from vulnerable low-lying areas, and encouraging a renewed focus on protecting agricultural lands and irrigation water resources at higher elevations.   


The project team developed and implemented a land subsidence, channel aggradation, and saltwater intrusion study coupled with community engagement activities to create a community vision for agricultural resilience in Snohomish County. This project complements and leverages several ongoing agricultural resilience and salmon habitat restoration project development initiatives in the Snohomish and Stillaguamish estuaries and watersheds, including: 1) restoration project development in the Snohomish and Stillaguamish floodplains; 2) flood modeling, groundwater modeling, and creation of several decision support tools to inform agricultural sustainability and resilience planning and land use decision-makers. The information created through this project and the complementary ongoing initiatives are expected to inform development of estuary floodplain salmon habitat and agricultural land management strategies by the Sustainable Lands Strategy. The Grantee will leverage several existing grant awards that fund hydrologic and hydraulic modeling, sea level rise modeling, and development of decision support tools.
The project team developed and implemented a land subsidence, channel aggradation, and saltwater intrusion study coupled with community engagement activities to create a community vision for agricultural resilience in Snohomish County. This project complements and leverages several ongoing agricultural resilience and salmon habitat restoration project development initiatives in the Snohomish and Stillaguamish estuaries and watersheds, including: 1) restoration project development in the Snohomish and Stillaguamish floodplains; 2) flood modeling, groundwater modeling, and creation of several decision support tools to inform agricultural sustainability and resilience planning and land use decision-makers. The information created through this project and the complementary ongoing initiatives are expected to inform development of estuary floodplain salmon habitat and agricultural land management strategies by the Sustainable Lands Strategy.  


*[https://secure.rco.wa.gov/prism/search/projectsnapshot.aspx?ProjectNumber=16-2270 PRISM Snapshot]
*[https://secure.rco.wa.gov/prism/search/projectsnapshot.aspx?ProjectNumber=16-2270 PRISM Snapshot]

Revision as of 13:16, 30 April 2020


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This project will investigate sea level rise impacts on saltwater intrusion and explore methodologies to detect and quantify land subsidence and river channel aggradation changes that may impact agricultural viability now and in future climate conditions; this work will inform the Snohomish Conservation District's Snohomish Agriculture Resilience Plan and is funded by the The ESRP Learning Program.

The Snohomish Conservation District led a collaborative effort with the Snohomish Sustainable Lands Strategy, UW Climate Impacts Group, Cardno, and NOAA to evaluate how climate change, sea level rise, and vertical land movement are likely to affect agricultural systems in the tidally influenced Lower Snohomish and Lower Stillaguamish river floodplains. This information supports estuary restoration as an appropriate response to protecting regional agricultural viability under sea level rise by describing the value of a strategic retreat from vulnerable low-lying areas, and encouraging a renewed focus on protecting agricultural lands and irrigation water resources at higher elevations.

The project team developed and implemented a land subsidence, channel aggradation, and saltwater intrusion study coupled with community engagement activities to create a community vision for agricultural resilience in Snohomish County. This project complements and leverages several ongoing agricultural resilience and salmon habitat restoration project development initiatives in the Snohomish and Stillaguamish estuaries and watersheds, including: 1) restoration project development in the Snohomish and Stillaguamish floodplains; 2) flood modeling, groundwater modeling, and creation of several decision support tools to inform agricultural sustainability and resilience planning and land use decision-makers. The information created through this project and the complementary ongoing initiatives are expected to inform development of estuary floodplain salmon habitat and agricultural land management strategies by the Sustainable Lands Strategy.

Subsidence and Aggradation[edit]

Subsidence refers to the downward sinking of the ground surface. Subsidence of agricultural lands can occur from the lack of sediment inputs to the floodplain, soil compaction, groundwater withdrawals, and decomposition of soil organics. Aggradation refers to the rising of the ground surface and, in this instance, refers to the accumulation of sediment within the river channel. Aggradation can increase the risk of flooding because it decreases the capacity of the river to carry flood volumes. Subsidence contributes to drainage issues in agricultural fields and can increase the risk of levee failure through settling and shifting. Therefore, aggradation within the river channel and subsidence of adjacent farmland can increase the flood and drainage impacts to some agricultural areas.

In order to study whether land subsidence and aggradation is affecting agriculture in the Snohomish and Stillaguamish River floodplains, the Conservation District contracted Cardno to conduct subsidence and aggradation studies for each watershed. To evaluate subsidence, Cardno re-surveyed elevations in areas that have been surveyed in the past, including monuments and benchmarks, roads, agricultural lands, and levees. The study also involved analysis of the vertical difference between elevations from multiple LiDAR datasets. To evaluate aggradation, Cardno compared recent channel cross-sections to historical surveys, evaluating 48 cross-sections of the Stillaguamish River and its tributaries and 19 cross-sections of the Snohomish River.

The analysis of farmland subsidence for both the Stillaguamish and the Snohomish River floodplains indicated that some areas may be sinking 2-3 inches per decade due to cultivation of organic soils, although error rates in this study are high.

The analysis also showed that the Lower Stillaguamish River channel is aggrading, and this trend is likely to continue into the future. The Lower Snohomish River is not aggrading in general, but upper reaches (from the SR-9 bridge to the Skykomish River) show some aggradation.

Saltwater Intrusion[edit]

Agricultural areas located near marine waters can suffer from saltwater intrusion, which occurs when saline waters move into groundwater aquifers. In the Lower Stillaguamish and Snohomish River floodplains, groundwater with increased salinity due to saltwater intrusion could affect the growing conditions for crops if that salinity reaches root zones. Though salts are crucial plant nutrients, high concentrations of any one salt or many different salts can be toxic to plants. Sea level rise could increase saltwater intrusion into groundwater in these areas as the saltwater interface rises in relation to freshwater aquifers.

Areas closest to the shoreline are at the highest risk of saltwater intrusion. Areas within 5,000 feet of the shoreline are especially vulnerable, and areas within 10,000 feet could also experience increases over time.

Florence Island, in the Stillaguamish River estuary, already experiences saltwater intrusion above crop tolerance thresholds in patches, and those impacts are likely to increase in severity over the next 50 years. Increasing pumping could pull salty water upward in the groundwater table.

In addition to analyzing groundwater levels and ponding, a groundwater study completed by Cardno also assessed the effect of sea level rise on saltwater intrusion into shallow groundwater. Cardno measured salinity levels in the wells drilled for the groundwater level study, as well as analyzed data from partner’s wells. Salinity impacts are measured in millisiemens per centimeter (mS/cm), a metric that measures conductivity values as a surrogate to salinity. Based on the salt tolerance of crops most commonly grown in the Lower Stillaguamish and Snohomish River floodplains (corn, grass, beets, spinach, and cabbage) and the depth of the wells used in the study, it was assumed that 3 mS/cm would best indicate potential impacts of saltwater intrusion on agricultural production. The response of plants to 0-2 mS/cm is mostly negligible, while sensitive plants can experience yield impacts with 2-4 mS/cm. Most plants would be restricted by 4-8 mS/cm, and only tolerant plants can grow under conditions with 8 mS/cm or more.

In the Lower Stillaguamish, existing conductivity measurements at wells within 1,000 feet of Hatt Slough showed a range of 0.1 to 6.7 mS/cm in late August 2016. These readings suggest that crops in the lower estuary may already be stressed by existing salinity conditions. Farmers in this area confirm that this is true in patches, but that most land is still highly productive. Data suggests that rising sea levels of one foot will increase conductivity measurements by approximately 1 mS/cm in the groundwater of farms near the coast.

Partners and Roles[edit]

  • Snohomish Conservation District - Project Lead
  • Cardno - Technical Study Consultant
  • UW Climate Impacts Group - Technical Collaborator
  • UW Crustal Deformation Group - Technical Collaborator