Antecedent Precipitation Tool (APT) Version 1.0 : technical and user guide

Abstract

This document provides an overview of the technical components of the Antecedent Precipitation Tool (APT) and a user guide for using the APT. The APT is an automation tool that the US Army Corps of Engineers (USACE) developed to facilitate the comparison of antecedent or recent precipitation conditions for a given location to the range of normal precipitation conditions that occurred during the preceding 30 yr¹. In addition to providing a standardized methodology to evaluate normal precipitation conditions (“precipitation normalcy”), the APT can assess the presence of drought conditions and the approximate dates of the wet and dry seasons for a given location.

Case study of continental‐scale hydrologic modeling's ability to predict daily streamflow percentiles for regulatory application

“…The APT was originally developed to automate the evaluation of precipitation and climatology in wetland delineations, delivering an estimate of precipitation normalcy known as an Antecedent Precipitation Score and Condition (below normal, normal, or above normal) while querying nClimDiv for monthly Climate Division estimates of the PDSI (Gutenson & Deters, 2022). The APT estimates the Antecedent Precipitation Score and Condition by evaluating 30‐day rolling totals of precipitation using the U.S. Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS) Engineering Field Handbook weighting factors (Gutenson & Deters, 2022; NRCS, 1997). This method is known as the “combined method”.…”

Evaluation of climatic and hydroclimatic resources to support the US Army Corps of Engineers Regulatory Program

“…As detailed in Gutenson and Deters (2022), α is a single value that ranges from 6 to 18 and is a weighted sum index comparing three 30-day rolling totals of precipitation on the observation date to historical totals over the preceding 30 years. The APT has two general ways of calculating α: a single-point or watershed analysis.…”

“…In an APT watershed analysis, the APT generates random sampling points throughout the upstream watershed, runs the equivalent of an APT singlepoint analysis at those upstream watershed sampling points, and averages the resulting values of α to generate the APT watershed-analysis α. Each APT run also collects PDSI data archived on a monthly time step at the climate division level in the nClimDiv data set (NOAA 2014; Vose et al 2014;Gutenson and Deters 2022). The PDSI is a cumulative index of water balance that tends to range between positive and negative 10 (NCAR 2020; Palmer 1965).…”

“…USACE developed the predecessor of the APT to streamline and automate the evaluation of precipitation normalcy and other climatic variables to complete wetland delineations whenever it was necessary to assess the following site-specific conditions: (1) dry season, (2) drought conditions, (3) lower than normal antecedent precipitation, or (4) greater than normal antecedent precipitation (USACE 2008). The APT automates an existing methodology for evaluating 30-day rolling totals of precipitation using the Engineering Field Handbook's weighting factors (NRCS 1997), which are useful for evaluating normal precipitation conditions (i.e., precipitation normalcy) for individual dates at an individual site (Sprecher and Warne 2000;Gutenson and Deters 2022).…”

“…As defined by the NWPR, typical year conditions are those that occur when precipitation and other climatic variables are within the normal periodic range (i.e., between the 30th and 70th percentiles) for the geographic area of the applicable aquatic resource based on a rolling 30year period. The NWPR requires an evaluation of typical year conditions when determining whether certain aquatic resources are jurisdictional (Gutenson and Deters 2022). Some locations in the US have ample data to determine normal climatic conditions as needed to assess typical high and low flow conditions or surface water connections in a typical year.…”