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Late Season Heat Flux and Water Distribution in Summer-fallowed Haploxerolls¹

ABSTRACT

The depth position and supply of water are especially critical soil management goals for fall-sown crops in a semiarid climate such as the Pacific Northwest of the USA where precipitation during late summer and early fall is not a reliable source of water for germination. Soil heat flux and water distribution were measured from 1 August to 1 November in a summer-fallowed soil which was cultivated several times with a rodweeder to create a loose soil mulch layer. Soil heat flux in the top 60 cm was measured using a combination of the temperature gradient method at 35 cm and calorimetry to obtain the flux at 1 cm. Daily soil heat energy at the 1-cm depth ranged from 710 kJ/m² on 2 August during clear weather (and with a dry soil mulch) to – 300 kJ/m² on 3 October, a clear day following a 30 mm rain. An additional daily heat energy of about 1000 kJ/m² was used on 2 August to evaporate 0.42 mm of water; on 3 October, the estimated additional daily heat energy to evaporate 0.98 mm of water was 2400 kJ/m². Soil water evaporation was measured during a wet and dry late summer season when the 0- to 9-cm mulch layer had respective average water contents of 0.15 and 0.05 cm³/cm³. Evaporation rates for comparable time periods during the dry year were generally one-third of the evaporation rate measured during the wet year. Light rain during late summer and early fall did not add significantly to stored soil water, because recharge and evaporation occurred mainly above the seedzone depth. Soil water evaporation rate during the wet year did not change appreciably from early September to early October even though potential evaporation declined 40%; this comparison period had similar rainfall amounts and antecedent soil moisture. When there were no late season rains to consolidate the mulch layer, the evaporation rate during September and October ranged near 0.15 mm/d. Soil water conservation benefits of the soil mulch disappear following rains that consolidate the mulch layer.

¹ Contribution from the USDA-ARS, Columbia Plateau Conservation Research Center, Pendleton, OR 97801.

² Soil Scientists and Computer Specialist, USDA-ARS, Columbia Plateau Conservation Research Center, Pendleton, OR 97801. Second author now located in the Soil & Water Conserv. Res. Unit, Soil Science Dep., Univ. of Minnesota.

Received for publication January 15, 1985. Accepted for publication April 2, 1985.