Soil moisture exerts a strong influence on ecology and energy balance of the environment. Its high variability in space and time, however, makes it difficult to measure accurately. Predicting the dynamics of soil moisture is also challenging because of the complex relationship between soil hydraulic properties and other physical characteristics. In this study, the links between soil structure and land surface characteristics with soil hydrology were determined using machine learning, unmanned aircraft system (drone), and observations from long-term conservation agriculture management.

The saturated hydraulic conductivity is an essential property of soil that determines the fate of soil moisture. Machine learning models were developed that predict saturated hydraulic conductivity with significantly improved accuracy than current models. The impact of soil structural changes on hydraulic conductivity was further investigated using these models. Soil moisture across a grassland surface soil was interpreted from multispectral imagery taken by a drone using machine learning-based methods. The interpretations had reasonable accuracy and were used to describe how land surface variables relate to surface soil moisture across a landscape. Soils that have been under different tillage and cover crop systems for the past 18-years were studied and compared to determine the implications of management on soil structure. Conventional measures of soil hydraulic properties and numerical simulations were applied to investigate the implications on soils' capacity to capture and store water.

Fire is a common ecosystem perturbation that affects many soil physical and chemical properties and soil organic matter (SOM). In my Master's thesis, I investigated the effect of combustion temperatures on the physical and chemical properties of five soils from an elevation transect that spans from 210 to 2865 m.a.s.l. along the Western slope of the Sierra Nevada. All soils were formed on a granitic parent material under either oak woodland, oak/mixed-conifer forest, mixed-conifer forest or subalpine mixed-conifer forest ecosystem. Soils show significant differences in SOM content and mineralogy owing to the effects of climate on soil development. Soils from 0 to 5 cm depth were combusted in a muffle furnace at six different temperatures within major fire intensity classes (150, 250, 350, 450, 550 and 650ºC). I determined the effects of combustion temperature on aggregation; specific surface area; pH; mineralogy; cation exchange capacity; carbon (C) and nitrogen (N) content and ¹³C and ¹⁵N isotopic composition in bulk and in separate aggregate sizes; and quality of SOM trough infrared spectroscopy. Among other things, I found significant reduction of total C and N, accumulation of aromatic carbon functional groups, and loss of aggregation with implication to loss of protection of C as the combustion temperature increases. My findings demonstrate that most significant changes in the soils physical and chemical properties occur around 350ºC. Findings from this study are critical for estimating the amount and rate of change in C and N loss, and other essential soil properties that can be expected from topsoils exposed to different intensity fires.

To meet the requirements of California's Sustainable Groundwater Management Act, there is a critical need for crop production strategies with less reliance on irrigation from surface and groundwater sources. One strategy for improving agricultural water use efficiency is reducing tillage and maintaining residues on the soil surface. We evaluated high residue no-till versus standard tillage in the San Joaquin Valley with and without cover crops on the yields of two crops, garbanzo and sorghum, for 4 years. The no-till treatment had no primary or secondary tillage. Sorghum yields were similar in no-till and standard tillage systems while no-till garbanzo yields matched or exceeded those of standard tillage, depending on the year. Cover crops had no effect on crop yields. Soil cover was highest under the no-till with cover crop system, averaging 97% versus 5% for the standard tillage without cover crop system. Our results suggest that garbanzos and sorghum can be grown under no-till practices in the San Joaquin Valley without loss of yield.

A long-term annual crop study in Five Points, California, shows that the combined use of no-tillage, surface residue retention, and cover crops improves soil health compared to conventional practices common to the region. Several chemical, biological, and physical soil health indicators were improved when these practices were combined. Our data suggest that farmers stand to gain multiple synergistic benefits from the integrated use of these practices by increasing soil structural stability, water infiltration and storage, and agroecosystem biodiversity, and improving the efficiencies of the carbon, nitrogen, and water cycles of their production systems.