Evolution of Arability of Land Use, Sacramento-San Joaquin Delta, California

Abstract

We used available data to estimate changes in land use and wet, non-farmable, and marginally farmable (WNMF) areas in the Delta from 1984 to 2012, and developed a conceptual model for processes that affect the changes observed. We analyzed aerial photography, groundwater levels, land-surface elevation data, well and boring logs, and surface water elevations. We used estimates for sea level rise and future subsidence to assess future vulnerability for the development of WNMF areas. The cumulative WNMF area increased linearly about 10-fold, from about 274 hectares (ha) in 1984 to about 2,800 ha in 2012. Moreover, several islands have experienced land use changes associated with reduced ability to drain the land. These have occurred primarily in the western and central Delta where organic soils have thinned; there are thin underlying mud deposits, and drainage ditches have not been maintained. Subsidence is the key process that will contribute to future increased likelihood of WNMF areas by reducing the thickness of organic soils and increasing hydraulic gradients and seepage onto the islands. To a lesser extent, sea level rise will also contribute to increased seepage onto islands by increasing groundwater levels in the aquifer under the organic soil and tidal mud, and increasing the hydraulic gradient onto islands from adjacent channels. WNMF areas develop from increased seepage under levees, which is caused by changing flow paths as organic soil thickness has decreased. This process is exacerbated by thin tidal mud deposits. Based primarily on projected reduced organic soil thickness and land-surface elevations, we delineated an additional area of about 3,450 ha that will be vulnerable to reduced arability and increased wetness by 2050.

Factors and Processes Affecting Levee System Vulnerability

“…For model runs with levees more than 4.5 m high and peat less than 3.5 m thick, 1.5 m of subsidence increased seepage flow by about 67%. This is consistent with data presented in Deverel et al (2015), which indicates that subsidence resulted in increased seepage-affected marginally or non-farmable areas 10-fold from 1984 to 2012 -a key factor was the presence of artesian conditions, which were delineated in most of the areas where there are organic soils ( Figure 6). …”

“…Land surface elevations where organic soils predominate (based on LiDAR data (CDWR 2007), and approximate delineation of artesian areas from Deverel et al (2015) http://dx.doi.org/10.15447/sfews.2016v14iss4art3 flooding of multiple islands and tracts : 6 in1980, 11 in 1983: 6 in1980, 11 in , 9 in 1986.…”

“…Most levees in the area of organic soils appear to contain, or be underlain to some degree by, organic materials (Atwater 1982 Deverel et al 2015). However, because the presentday artificial levees were mostly constructed on the overflow banks of the river and distributary sloughs, most rest on natural levee deposits (Figure 12) or on peat and mud deposits that interfinger in the subsurface, creating vertical interbeds of silt and sand with organic-rich material.…”

Implications for Greenhouse Gas Emission Reductions and Economics of a Changing Agricultural Mosaic in the Sacramento–San Joaquin Delta

“…Subsidence and associated seepage is a key factor that contributes to decreasing agricultural sustainability and levee vulnerability in the Delta (Deverel et al 2015;Deverel et al 2016b). Managed wetlands stop and reverse the effects of subsidence (Miller et al 2008).…”

Solutions for subsidence in the California Delta, USA, an extreme example of organic-soil drainage gone awry

“…Drainage of the peatlands, which began in the mid-19th century, resulted in the loss of about one-half of this volume. This degradative process is on-going (Deverel et al, 2016a) and has resulted in elevations of Delta islands as low as 9 m below sea level (Fig. 1).…”