Crop growth and development effects on surface albedo for maize and cowpea fields in Ghana, West Africa

Abstract

The albedo (α) of vegetated land surfaces is a key regulatory factor in atmospheric circulation and plays an important role in mechanistic accounting of many ecological processes. This paper examines the influence of the phenological stages of maize (Zea mays) and cowpea (Vigna unguiculata) fields on observed albedo at a tropical site in Ghana. The crops were studied for the first and second planting dates in the year 2002. Crop management was similar for both seasons and measurements were taken from 10 m×10-m plots within crop fields. Four phenological stages were distinguished: (1) emergence, (2) vegetative, (3) flowering, and (4) maturity. α measured from two reference surfaces, short grass and bare soil, were used to study the change over the growing seasons. Surface α was measured and simulated at sun angles of 15, 30, 45, 60, and 75°. Leaf area index (LAI) and crop height (CH) were also monitored. Generally, α increases from emergence to maturity for both planting dates in the maize field but slightly decreases after flowering in the cowpea field. For maize, the correlation coefficient (R) between α and LAI equals 0.970, and the R between α and CH equals 0.969. Similarly, for cowpea these Rs are 0.988 and 0.943, respectively. A modified albedo model adequately predicted the observed αs with an overall R>0.860. The relative difference in surface α with respect to the α values measured from the two reference surfaces is discussed. Data presented are expected to be a valuable input in agricultural water management, crop production models, eco-hydrological models and in the study of climate effects of agricultural production, and for the parameterization of land-surface schemes in regional weather and climate models.