Image-Based Evaluation of Spray Deposition Using Water-Sensitive Papers: Metrics, Limitations, and Practical Implications

Water-sensitive papers (WSPs) are widely used for spray deposition assessment because they are inexpensive, simple to use, and suitable for field conditions. Combined with image analysis, they provide quantitative information on spray coverage and indirect insight into deposition structure. However, their interpretation is often oversimplified, particularly when percent coverage is treated as the sole indicator of spray quality. This paper presents a critical methodological review of image-based evaluation of spray deposition using WSPs, with emphasis on coverage-related metrics, spatial descriptors, droplet size estimation, and the main sources of uncertainty affecting their interpretation. The review also positions WSPs relative to other spray characterization techniques and discusses their practical role as proxy-based tools rather than direct measurement instruments. Representative WSP samples from previous field experiments are used exclusively to illustrate typical processing steps and methodological pitfalls, not to report new experimental results. In addition, the paper summarizes major segmentation approaches, discusses the interpretative value of selected deposition descriptors, and formulates practical recommendations for image acquisition, binarization, metric selection, sample exclusion, and reporting practice. It is concluded that WSP-based image analysis is most valuable for comparative and diagnostic assessment of spray deposition, provided that its methodological constraints are explicitly recognized and consistently reported.