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The moisture and thermal regimes of a bare soil in the lower Fraser Valley during spring

University of British Columbia

Half-hourly measurements of the surface energy balance components and soil temperatures were made at Agassiz, B.C., in the spring and early summer of 1978 at two adjacent bare-soil sites, one of which was culti-packed, while the other was disc-harrowed. The latent and sensible heat flux densities were measured using the energy balance/Bowen ratio technique with reversing psychrometer units. Soil surface heat flux density, G₀, was calculated using the null-alignment procedure from half-hourly measurements of soil temperature at 30 depths down to 1 m and volumetric soil heat capacity calculated from measurements of bulk density, organic matter fraction, and moisture content measured gravimetrically at least every 2 days. The bulk density of the upper 10 cm of soil was reduced 10 - 20% by the disc-harrowing. Net radiation was reduced by 7% and evaporation by 40% at the disc-harrowed site during a 16-day almost-rainless period. Surface drying was greater at the disc-harrowed site, which in conjunction with the lower bulk density led to a greater reduction in near-surface volumetric soil heat capacity and thermal conductivity. The daily average of G₀ was not affected by either the tillage or surface drying, although its diurnal amplitude was reduced by the disc-harrowing. Both daily and daytime averages of near-surface soil temperature were higher and nighttime averages slightly lower at the disc-harrowed site. Surface drying increased the diurnal amplitudes of near surface soil temperatures, particularly for the disc-harrowed site. The effects of the disc-harrowing and surface drying on the soil thermal regime were mostly attributed to the resulting reductions in near-surface thermal properties. The relative increase in atmospheric admittance that occurred with surface drying exceeded the corresponding decrease in soil admittance at both sites. It was concluded that the increase in atmospheric admittance, which was attributed to the greater atmospheric instability under the resulting stronger lapse rates, must be included when partitioning available energy, at the earth's surface. Daily and daytime averages of G₀ at each site could be expressed as simple functions of either the solar irradiance alone or net radiation and some measure of near-surface soil moisture status. Nighttime average G₀ at both sites could be expressed as a function of a cloudiness ratio based upon the daytime average of solar irradiance. To calculate soil temperature, the exact solution to the equations of heat transfer for a homogeneous finite layer overlying a homogeneous semi-infinite layer with G₀ as a boundary condition was derived. The theory was found to be useful in assessing the effects of tillage and drying on daily average temperature. However, for relatively wet sites such as at Agassiz, the derivation of a solution in which the variation of k and C with depth and time is better represented than by the simple two-layered model is desirable. The results showed that all methods that calculate soil temperature using G₀ as a boundary condition are sensitive to small systematic errors in G₀ over periods greater than 10 days. Calculation of diurnal variations of soil temperature using the harmonic solution to the two-layered model was tested. This procedure underestimated the diurnal variation of the surface temperature on cloudy days and overestimated on clear days when the soil surface was dry, particularly at the culti-packed site. An empirical equation developed by Idso's group at Phoenix, Arizona to calculate daily average evaporation rates during all 3 drying stages of a bare soil was tested and discussed on the basis of available evaporation theory. The results show that the Idso expression for potential evaporation rate did not apply at Agassiz due to differences in the advection regimes at the two locations. The Agassiz potential evaporation rate data was well represented by the Priestley-Taylor equation with a[sub PT] ("alpha") = 1.27 ± 0.1. It was concluded that Idso's equation for potential evaporation rate has no greater generality than the Priestley-Taylor or other such semi-empirical approach. The concept of expressing the stage III rates as proportional to the expression for potential evaporation rate worked marginally well at the culti-packed site and quite well at the disc-harrowed site. It was concluded that for soils with stage III rates much greater than 50% of potential evaporation rate more complete procedures are necessary for calculating evaporation rates during extended drying periods.

Text

2010-03-29

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http://hdl.handle.net/2429/22898

Soil Science

University of British Columbia

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