Luckily for wine lovers, the rain in Spain does not fall mainly on the plain. Enough moisture drops in the northern mountain areas and on the Mediterranean coast to produce some of the best red wine values in today's market.

Spain can claim a number of viticultural firsts. According to several authorities, the grape has flourished there longer than in any other Western European country - at least since Roman times and possibly as long ago as the Phoenicians. Spain can boast of the largest vine acreage in the world, and it has one of the oldest appellation-control systems on record.

Nonetheless, Spanish wines often are neglected. Perhaps it is the "reserva" system, which ages red wines in the wood much longer than in Bordeaux and thus causes a rounding off of the potential of the wine. Or maybe it is the shipper system of blending, which lacks the distinctive appeal of a "chateau" bottling. Whatever the reason, in the age of $10 Bordeauxs, Burgundies and California varietals, the reds of Spain at$3 to $4 are the best modestly priced wines available in Washington.

It is difficult to review Spanish red wines without starting at Rioja, the most famous natural wine region of the country. Rioja lies parallel to the French border in northern Spain. Roughly an 80-mile oval encompassing the River Ebro, Rioja is protected by mountains and blessed with judicious amounts of sunshine and rain. The westernmost section, divided into the Rioja Alta and Rioja Alavesa, produces the most refined wines, while the Rioja Baja to the east is more Mediterranean in character. The wine trade is centered in the cities of Haro, Ollauri and Logrono - placed names often found on bottle labels.

Rioja represents a unique blend of French and Spanish winemaking history. When phylloxera (a vine louse) struck French vineyards in 1968, many small growers fled to the Rioja to start over. There they combined local customs and new grape varieties with French vinification techniques to form the classic Rioja style.

Riojan red-wine grapes are produced by hundreds of small growers who sell their yields to a handful of "bodegas" or shippers. The bodega blends the wine and places its names on the bottle.Occassionally a vineyard name is added, but it should not be assumed that the contents all come from that specific location. The prevalent grape types include the Tempraniolo and Garnacha.

Quality depends on the reputation of the shipper rather than on strict appellation controls. It must be remembered that while broad geographical restrictions are enforced, there is more latitude for vintage dating. Blending is carried on not only within a vintage but among vintages. It seems that the more popular vintages always are available in quantity.

One recurring criticism of Riojas is the tendency to keep them for too long in the wood. Some of the best shippers age in the bottle but others continue the centuries-old practice of storing "reservas" in wood cooperage for five years or more before bottling. This weakens the wine and cuts short its developing life. For this reason, and others, the old Riojas seldom attain the heights of perfection or finesse of Bordeaux.

Not all Spanish reds come from Rioja. Near Barcelona, in the region of Panades, lie the holdings of the Torres family. Of all Spanish reds, the Torres Coronas is the most uniformaly delicious. Heavy in the mouth, robust, melow with age, the Coronas is my pick of all modestly priced red wines. At $2.49 to $2.99, it is a steal. The powerful Torres Sangre de Torro makes a remarkable sangria base and the Gran Reserva Torres selections are always worth a try.

What else to drink? One of my long-standing favorites is Marques de Murrieta. In the $4 range, this fine wine is smooth, rich and almost sweet in its finish. Only slightly less attractive are the 1973 Marques de Riscal at $3.99 and the 1974 Paternina Banda Azul at $3.59. Below this level there is an assortment of average wines in the $3 to $4 class - all very drinkable but with no outstanding attributes except Spanish warmth. The list might include Bilbainas Vina Zaco, Marques del Romeral, Vina Pomal, Cune Vina Real, Paternina Vina Vial, Bilbainas Rioja Edena, and Domecq Domain.

The reservas pose another problem. There is a great variation in the quality of individual bottles. Some very old reservas - the 1954 Murrieta, 1957 Riscal and 1954 Vina Bosconia - are extraordinary. The occasionally available Excelso Grand Reserva often displays a French character. Nonetheless, many reservas provide that unwelcome excitement that comes from not knowing if the results will be excellent or disappointing. Buy one and try it before commiting yourself to larger quantities.

A last word of warning about all Spanish wines. Care should be taken in purchasing since they do not move well in some stores and may not have been kept properly on the shelves. Dark corks are common. Allow the wine to breath at least a half hour before drinking and do not be alarmed if little sediment is present even in older bottles. This is normal for Spanish reds held in the wood and racked often.