Add waterdata.get_nearest_continuous helper (#239)

For each target timestamp, fetch the nearest continuous observation in
a single round trip. Builds a CQL OR-chain of per-target bracketed
windows, pipes it through ``get_continuous`` (which already auto-chunks
long filters across multiple sub-requests), then selects the single
observation closest to each target client-side.

Exists because the Water Data API matches a single-instant ``time=``
parameter exactly (10:30:31 returns zero rows on a 15-minute gauge),
does not implement ``sortby`` for arbitrary queryables, and does not
expose a ``T_NEAREST`` CQL function. The narrow-window + client-side
reduction is the one pattern that works today for multi-target
nearest lookups in one API call.

Tie handling is configurable via ``on_tie``:
  - "first" (default): keep the earlier observation
  - "last":  keep the later observation
  - "mean":  average numeric columns; set ``time`` to the target

Author: thodson-usgs

NEWS.md

@@ -1,3 +1,5 @@
+**04/23/2026:** Added `waterdata.get_nearest_continuous(targets, ...)` — for each of N target timestamps, fetches the single continuous observation closest to that timestamp in one HTTP round-trip (auto-chunked when the resulting CQL filter is long, via the facility added in #238). The helper is designed for workflows that pair many discrete-measurement timestamps with surrounding instantaneous data, which the OGC `time` parameter can't express since it only accepts one instant or one interval per request. Ties at window midpoints are resolved per a configurable `on_tie` ∈ {`"first"`, `"last"`, `"mean"`}; the default `window="PT7M30S"` matches a 15-minute continuous gauge.
+
 **04/22/2026:** Highlights since the `v1.1.0` release (2025-11-26), which shipped the `waterdata` module:
 
 - Added `get_channel` for channel-measurement data (#218) and `get_stats_por` / `get_stats_date_range` for period-of-record and daily statistics (#207).

dataretrieval/waterdata/__init__.py

@@ -19,6 +19,7 @@
     get_latest_continuous,
     get_latest_daily,
     get_monitoring_locations,
+    get_nearest_continuous,
     get_reference_table,
     get_samples,
     get_stats_date_range,
@@ -47,6 +48,7 @@
     "get_latest_continuous",
     "get_latest_daily",
     "get_monitoring_locations",
+    "get_nearest_continuous",
     "get_reference_table",
     "get_samples",
     "get_stats_date_range",

dataretrieval/waterdata/api.py

@@ -9,7 +9,7 @@
 import json
 import logging
 from io import StringIO
-from typing import get_args
+from typing import Literal, get_args
 
 import pandas as pd
 import requests
@@ -440,6 +440,239 @@ def get_continuous(
     return get_ogc_data(args, output_id, service)
 
 
+def get_nearest_continuous(
+    targets,
+    monitoring_location_id: str | list[str] | None = None,
+    parameter_code: str | list[str] | None = None,
+    *,
+    window: str | pd.Timedelta = "PT7M30S",
+    on_tie: Literal["first", "last", "mean"] = "first",
+    **kwargs,
+) -> tuple[pd.DataFrame, BaseMetadata]:
+    """For each target timestamp, return the nearest continuous observation.
+
+    Builds one bracketed ``(time >= t-window AND time <= t+window)`` clause
+    per target, joins them as a top-level CQL ``OR`` filter, and lets
+    ``get_continuous`` (with its auto-chunking) fetch every observation
+    that falls in any window. Then, per ``(monitoring_location_id, target)``
+    pair, picks the single observation with the smallest ``|time - target|``.
+
+    The USGS continuous endpoint matches ``time`` parameters exactly rather
+    than fuzzily, and it does not implement ``sortby`` for arbitrary fields;
+    this function is the single-round-trip way to ask "what reading is
+    nearest this timestamp?" for many timestamps at once.
+
+    Parameters
+    ----------
+    targets : list-like of datetime-convertible
+        Target timestamps. Naive datetimes are treated as UTC. Accepts a
+        list, ``pandas.Series``, ``pandas.DatetimeIndex``, ``numpy.ndarray``,
+        or anything ``pandas.to_datetime`` consumes.
+    monitoring_location_id : string or list of strings, optional
+        Forwarded to ``get_continuous``.
+    parameter_code : string or list of strings, optional
+        Forwarded to ``get_continuous``.
+    window : string or ``pandas.Timedelta``, default ``"PT7M30S"``
+        Half-window around each target, as an ISO 8601 duration
+        (``"PT7M30S"``, ``"PT15M"``, ``"PT1H"``, etc.). Also accepts
+        any other form ``pandas.Timedelta`` parses — ``HH:MM:SS``
+        (``"00:07:30"``), pandas shorthand (``"7min30s"``,
+        ``"450s"``), or a ``pd.Timedelta`` directly. See the
+        `pandas.Timedelta docs
+        <https://pandas.pydata.org/docs/reference/api/pandas.Timedelta.html>`_
+        for the full grammar.
+
+        Must be small enough that every target's window captures
+        roughly one observation at the service cadence. The default
+        matches a 15-minute continuous gauge; widen (e.g.
+        ``"PT15M"``) for irregular cadences or resilience to data
+        gaps.
+    on_tie : {"first", "last", "mean"}, default ``"first"``
+        How to resolve ties when two observations are exactly equidistant
+        from a target (which happens when the target falls at the midpoint
+        between grid points — e.g. target ``10:22:30`` for a 15-minute
+        gauge).
+
+        - ``"first"``: keep the earlier observation.
+        - ``"last"``:  keep the later observation.
+        - ``"mean"``:  average numeric columns; set the ``time`` column to
+          the target, since no real observation exists at the midpoint.
+
+    **kwargs
+        Additional keyword arguments forwarded to ``get_continuous``
+        (e.g. ``statistic_id``, ``approval_status``, ``properties``).
+        Passing ``time``, ``filter``, or ``filter_lang`` raises
+        ``TypeError`` — this function builds those itself.
+
+    Returns
+    -------
+    df : ``pandas.DataFrame``
+        One row per ``(target, monitoring_location_id)`` combination that
+        had at least one observation in its window. Rows are augmented
+        with a ``target_time`` column indicating which target they
+        correspond to. Targets with no observations in their window are
+        silently dropped.
+    md : :class:`~dataretrieval.utils.BaseMetadata`
+        Metadata from the underlying ``get_continuous`` call.
+
+    Notes
+    -----
+    *Window sizing and ties.* When ``window`` is exactly half the service
+    cadence, most targets' windows contain a single observation and
+    ``on_tie`` is moot. Ties arise only when a target sits exactly at the
+    window edge — rare in practice but possible. Setting ``window`` to a
+    full cadence (or larger) guarantees at least one observation per
+    target in steady state at the cost of more bytes per response.
+
+    *Why windowed CQL rather than sort+limit.* The API's advertised
+    ``sortby`` parameter would make this a one-liner per target (``filter``
+    by ``time <= t`` and ``limit 1``), but it is per-query — you would need
+    one HTTP round-trip per target. The CQL ``OR``-chain approach folds
+    all N targets into one request (auto-chunked when the URL is long).
+
+    Examples
+    --------
+    .. code::
+
+        >>> import pandas as pd
+        >>> from dataretrieval import waterdata
+
+        >>> # Pair three off-grid timestamps with nearby observations
+        >>> targets = pd.to_datetime(
+        ...     [
+        ...         "2023-06-15T10:30:31Z",
+        ...         "2023-06-15T14:07:12Z",
+        ...         "2023-06-16T03:45:19Z",
+        ...     ]
+        ... )
+        >>> df, md = waterdata.get_nearest_continuous(
+        ...     targets,
+        ...     monitoring_location_id="USGS-02238500",
+        ...     parameter_code="00060",
+        ... )
+
+        >>> # Widen the window for an irregular-cadence gauge
+        >>> df, md = waterdata.get_nearest_continuous(
+        ...     targets,
+        ...     monitoring_location_id="USGS-02238500",
+        ...     parameter_code="00060",
+        ...     window="PT30M",
+        ...     on_tie="mean",
+        ... )
+    """
+    _check_nearest_kwargs(kwargs, on_tie)
+    targets = pd.DatetimeIndex(pd.to_datetime(targets, utc=True))
+    window_td = pd.Timedelta(window)
+
+    if len(targets) == 0:
+        raise ValueError("targets must contain at least one timestamp")
+
+    filter_expr = _build_window_or_filter(targets, window_td)
+    df, md = get_continuous(
+        monitoring_location_id=monitoring_location_id,
+        parameter_code=parameter_code,
+        filter=filter_expr,
+        filter_lang="cql-text",
+        **kwargs,
+    )
+    if df.empty:
+        return _empty_nearest_result(df), md
+
+    df = df.assign(time=pd.to_datetime(df["time"], utc=True))
+    site_groups = (
+        df.groupby("monitoring_location_id", sort=False)
+        if "monitoring_location_id" in df.columns
+        else [(None, df)]
+    )
+
+    selected = [
+        row
+        for _, site_df in site_groups
+        for target in targets
+        if (row := _pick_nearest_row(site_df, target, window_td, on_tie)) is not None
+    ]
+    if not selected:
+        return _empty_nearest_result(df), md
+    return pd.DataFrame(selected).reset_index(drop=True), md
+
+
+_VALID_ON_TIE = ("first", "last", "mean")
+
+
+def _check_nearest_kwargs(kwargs: dict, on_tie: str) -> None:
+    """Reject kwargs the helper owns; validate ``on_tie``."""
+    for forbidden in ("time", "filter", "filter_lang"):
+        if forbidden in kwargs:
+            raise TypeError(
+                f"get_nearest_continuous constructs its own {forbidden!r}; "
+                "do not pass it directly"
+            )
+    if on_tie not in _VALID_ON_TIE:
+        raise ValueError(f"on_tie must be one of {_VALID_ON_TIE}; got {on_tie!r}")
+
+
+def _build_window_or_filter(targets: pd.DatetimeIndex, window_td: pd.Timedelta) -> str:
+    """Build the CQL OR-chain of ``time >= ... AND time <= ...`` windows.
+
+    ``get_continuous`` auto-chunks the result if the full URL would
+    exceed the server's length limit, so this is always safe to build
+    as one string even for many targets.
+    """
+    return " OR ".join(
+        f"(time >= '{(t - window_td).strftime('%Y-%m-%dT%H:%M:%SZ')}' "
+        f"AND time <= '{(t + window_td).strftime('%Y-%m-%dT%H:%M:%SZ')}')"
+        for t in targets
+    )
+
+
+def _pick_nearest_row(
+    site_df: pd.DataFrame,
+    target: pd.Timestamp,
+    window_td: pd.Timedelta,
+    on_tie: str,
+) -> pd.Series | None:
+    """Return the single row within ``window_td`` of ``target``, or ``None``.
+
+    Resolves ties (two rows equidistant from ``target``) per ``on_tie``.
+    The returned row carries a ``target_time`` column identifying which
+    target it was selected for.
+    """
+    in_window = site_df[
+        (site_df["time"] >= target - window_td)
+        & (site_df["time"] <= target + window_td)
+    ]
+    if in_window.empty:
+        return None
+    deltas = (in_window["time"] - target).abs()
+    candidates = in_window[deltas == deltas.min()].sort_values("time")
+
+    if len(candidates) == 1 or on_tie == "first":
+        row = candidates.iloc[0].copy()
+    elif on_tie == "last":
+        row = candidates.iloc[-1].copy()
+    else:  # "mean" — synthesize a row whose numeric cols are averaged and
+        # whose ``time`` is the target (no real observation sits at the midpoint).
+        row = candidates.iloc[0].copy()
+        for col in candidates.select_dtypes("number").columns:
+            row[col] = candidates[col].mean()
+        row["time"] = target
+
+    row["target_time"] = target
+    return row
+
+
+def _empty_nearest_result(template: pd.DataFrame | None = None) -> pd.DataFrame:
+    """Empty frame with a ``target_time`` column, for no-match cases.
+
+    When ``template`` is provided, preserve its columns/dtypes so the
+    returned frame matches the shape of a real ``get_continuous``
+    response.
+    """
+    base = pd.DataFrame() if template is None else template.iloc[0:0].copy()
+    base["target_time"] = pd.Series(dtype="datetime64[ns, UTC]")
+    return base
+
+
 def get_monitoring_locations(
     monitoring_location_id: list[str] | None = None,
     agency_code: list[str] | None = None,