Estimating the mass wetness of Spanish arid soils from lightness measurements

The colour of a soil changes with its water content. This paper investigates the ability of spectrophotometric colour measurement to predict soil mass wetness (w). We looked at the CIELAB parameters (L⁎, C⁎ab, hab) and spectral profile of 76 A soil-horizons from south-eastern Spain to (i) group them by colour, (ii) calibrate within each group the relationships between water content and colour in disturbed and undisturbed samples, and (iii) test the validity of predictive models. Four groups of differently coloured soils were selected from the reflectance curves. Models constructed only with L⁎ (lightness) from the dryness state to the water content at − 33/− 10 kPa explained the greatest variation in w (R2 = 0.77–0.97), regardless of the soil colour or sample type. The decrease in soil lightness with increasing water content was noted mainly at − 1500 kPa, − 400/− 100 kPa, and − 33/− 10 kPa potentials. At intermediate potentials, however, L⁎ did not strongly correlate with w, especially in undisturbed samples, which showed a greater colour variability. Although the predictive models did not give estimates of w with high enough precision (mean relative errors 25.3–56.6%), the measured values below − 1500 kPa, between − 1500 and − 100 kPa, and above − 100 kPa were predicted within the same interval of water potential. The results indicate that predictions of the dryness condition, presence of plant-available water and wetness near to field capacity, but not the specific water content, can be made with reasonable confidence in any soil by using the models calibrated in other soil of similar colour.