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Subsidence of organic soils in the Sacramento-San Joaquin Delta threatens sustainability of the California (USA) water supply system and agriculture. Land-surface elevation data were collected to assess present-day subsidence rates and evaluate rice as a land use for subsidence mitigation. To depict Delta-wide present-day rates of subsidence, the previously developed SUBCALC model was refined and calibrated using recent data for CO2 emissions and land-surface elevation changes measured at extensometers. Land-surface elevation change data were evaluated relative to indirect estimates of subsidence and accretion using carbon and nitrogen flux data for rice cultivation. Extensometer and leveling data demonstrate seasonal variations in land-surface elevations associated with groundwater-level fluctuations and inelastic subsidence rates of 0.5–0.8 cm yr–1. Calibration of the SUBCALC model indicated accuracy of ±0.10 cm yr–1 where depth to groundwater, soil organic matter content and temperature are known. Regional estimates of subsidence range from <0.3 to >1.8 cm yr–1. The primary uncertainty is the distribution of soil organic matter content which results in spatial averaging in the mapping of subsidence rates. Analysis of leveling and extensometer data in rice fields resulted in an estimated accretion rate of 0.02–0.8 cm yr–1. These values generally agreed with indirect estimates based on carbon fluxes and nitrogen mineralization, thus preliminarily demonstrating that rice will stop or greatly reduce subsidence. Areas below elevations of –2 m are candidate areas for implementation of mitigation measures such as rice because there is active subsidence occurring at rates greater than 0.4 cm yr–1.

The object of the present study was to investigate the hypothesis that a low systemic blood pressure might be associated with so-called low tension glaucoma: the rather unexpected result was an association between open-angle glaucoma and vascular hypertension. Theoretically, the pressure in the ophthalmic artery as shown by ophthalmodynamometry is likely to be a better index of blood pressure in arteries in the eyeball than the systemic blood pressure measured at the upper arm; however, in practice, we have the impression that the latter estimate has less observer error and so it has been chosen for this study. A good correlation seems to exist between ophthalmic artery and brachial artery pressure (Smith and Cogan, I959; Johnson and Drance, i 968a). I'he relationship between ischaemia of the optic nerve and retina, blood pressure, and glaucoma has been studied by several authors in various ways:(a) Optic nerve ischaemia and glaucoma Schnabel (i885) introduced the term "cavernous optic atrophy" and Wolff (1947) agreed that the condition was the result of ischaemia. Duke-Elder (i962) also considers arteriosclerosis, etc., to be important in open-angle glaucoma.(b) Low tension glaucoma and low blood pressure Weekers (1942) and Sjogren (1946) noted that patients with low tension glaucoma often have low blood pressure.(c) Lowering of blood pressure and increasedfield loss These have been observed in glaucoma by several authors, for example Harrington (I959) and Lobstein, Bronner, and Nordmann (i960).(d) Glaucoma, blood pressure (including perfusion pressure), and atherosclerosis (i) LOW BLOOD PRESSURE Sachsenweger (I963) and Ebner (I967) found a more rapid deterioration in the visual fields in glaucoma patients with low blood pressure than in those with either high or normal blood pressure. Lobstein and Herr ( i 966) showed that, in glaucoma, field loss was greater the lower the blood pressure.

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From 2000 to 2003 we quantified drain flow, drainand ground-water chemistry and hydrogeologic conditions on Twitchell Island in the Sacramento-San Joaquin Delta. The primary objective was to quantify processes affecting organic carbon concentrations and loads in agricultural drainage water. We collected physical and chemical data in southern and northern areas: TN and TS, respectively. Corn grew in both areas during the spring and summer. The peat soils in the TN area are more decomposed than those in the TS area. Results elucidate processes affecting drain flow and concentrations under varying hydrologic conditions. During May through November, groundwater flows from the permanently saturated zone to drainage ditches, and the resulting average drainagewater quality and dissolved organic carbon (DOC) concentration was similar to the groundwater; the median DOC loads in the TN and TS study areas ranged from 9 to 27 g C/ha-day. The major ion chemistry and stable isotope data confirmed that groundwater was the primary source of drainflow. In contrast, during December through April the drainwater is supplied from the shallow, variably saturated soilzone. The DOC concentrations, major-ion chemistry and stable isotope data indicate the shallow-zone water is partially evaporated and oxidized. Higher flows and DOC concentrations during these months result in higher median DOC loads, which ranged from 84 to 280 g C/ha-day.During December through April, increasing groundwater levels in the shallow peat layers and mobilization of organic carbon result in high drain flow and increased trihalomethane precursor concentrations and loads. On a per mass DOC basis, drain water collected during high flow periods is less likely to form THMs than during low flow periods. However, the high flows and subsequent high concentrations contribute to substantially higher trihalomethane precursor and DOC loads. KEYWORDSagricultural drainage, organic carbon, drinking water quality, disinfection byproducts, organic soils